Chapter 5 Complex Numbers and Quadratic Equations Solutions – NCERT Class 11 Mathematics

Exercise 5.1

Express each of the complex numbers given in the Exercises 1 to 10 in the form a + ib

\begin{equation} \textbf{1.} \left( 5i \right) \left( \frac{-3i}{5} \right) \\ = \frac{- 15i^{2}}{5} \\ = – 3i^{2} \\ \text{Replacing } i^{2} = – 1 \\ \implies – 3(-1) = 3 \end{equation}

As (5i)(- 3i/5) just simplifies to 3 which is just a Real Number thus this complex number can be written as 3 + i0 as it doesn’t have any imaginary part.


simplifying i^9 + i^19

Hence i9 + i19 = 0 which can be written as 0 + i0 in the form a + ib


3. i-39

simplifying i^-39

Thus value of i-39 = i which can be written as 0 + i in the form a + ib


4. 3(7 + 7i) + i(7 + 7i)

3(7 + 7i) + i(7 + 7i) = 21 + 21i + 7i + 7i2 = 21 + 28i + 7i2

3(7 + 7i) + i(7 + 7i) = 21 + 28i + 7i2
Replacing i2 = – 1

3(7 + 7i) + i(7 + 7i) = 21 + 28i + 7(- 1) = 21 + 28i -7 = 14 + 28i

3(7 + 7i) + i(7 + 7i) = 14 + 28i

Thus 3(7 + 7i) + i(7 + 7i) can be written as 14 + 28i in a + ib form


5. (1 – i) – (- 1 + 6i)

(1 – i) – (- 1 + 6i) = 1 – i + 1 – 6i
= 2 – 7i

⇒ (1 – i) – (- 1 + 6i) = 2 – 7i

Thus (1 – i) – (- 1 + 6i) can be written as 2 – 7i in a + ib form


6. (1/5 + 2i/5) – (4 + 5i/2)

simplifying 1/5 + 2i/4 - 4 - 5i/2

Thus (1/5 + 2i/5) – (4 + 5i/2) can be written as – 19/5 + (-19i/10) in a + ib form


simplifying [(1/3 + 7i/3) + (4 + i3)] - (- 4/3 + i) in a + ib form

8. (1 – i)4

In order to write (1 – i)4 in a + ib form let’s first simplify this.

simplifying (1 - i)^4

9. (1/3 + 3i)3

simplifying (1/3 + 3i)^3

Thus (1/3 + 3i)3 can be written as – 242/9 – 26i in the a + bi form


10. (- 2 – 1/3i)3

simplifying (- 2 - 1/3i)^3

Thus (- 2 – 1/3i)3 can be written as – 22/3 + 107i/27 in the a + ib form


Find the multiplicative inverse of each of the complex numbers given in the Exercises 11 to 13.

11. 4 – 3i

Multiplicative Inverse of any Complex Number of form a + ib is equals to 1/a + ib
⇒ Multiplicative inverse of 4 – 3i is equals to 1/4 – 3i

simplifying 1/(4 - 3i)

Thus Multiplicative Inverse of 4 – 3i is 4/25 + 3i/25


12. √5 + 3i

Multiplicative Inverse of any Complex Number of form a + ib is equals to 1/a + ib
⇒ Multiplicative inverse of √5 + 3i is equals to 1/(√5 + 3i)

simplifying 1/(√5 + 3i)

Thus Multiplicative Inverse of √5 + 3i is √5/14 – 3i/14


13. – i

Multiplicative Inverse of any Complex Number of form a + ib is equals to 1/a + ib
⇒ Multiplicative inverse of – i is equals to 1/- i

simplifying - 1/i

Thus Multiplicative Inverse of – i is i


14. Express the following expression in the form of a + ib : (3 + i√5)(3 – i√5)/[(√3 + √2i) – (√3 – i√2)]

simplifying (3 + i √5)(3 - i √5)/((√3 + √2i) - (√3 - i √2))

Thus (3 + i√5)(3 – i√5)/[(√3 + √2i) – (√3 – i√2)] can be written as 0 – 7√2i/2 in the a + bi form


Exercise 5.2

Find the modulus and the arguments of each of the complex numbers in Exercises 1 to 2.

1. z = – 1 – i √3

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 – i √3 with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = – √3

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (-1)2 + (- √3)2 = 1 + 3 = 4

⇒ r2Cos2θ + r2Sin2θ = 4

r2(Cos2θ + Sin2θ) = 4

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 4

r2 = 4

r = ± 2
Thus r = 2 or -2 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = 2

Hence Modulus of Complex Number z = – 1 – i √3 is 2


Now let’s find out Argument of the Complex Number z = – 1 – i √3

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 – i √3 with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = – √3

Replacing r = 2 in these equations
2Cosθ = – 1
2Sinθ = – √3

⇒ Cosθ = – 1/2
⇒ Sinθ = – √3/2

⇒ θ = 4𝜋/3
As both Cosine and Sine are negative thus θ must lie in the third quadrant, but by definition Argument of a Complex Number lie in range − 𝜋 < 𝜃 ≤ 𝜋.

So θ if taken in reverse direction from fourth quadrant of x-y plane will be – (2𝜋 – 4𝜋/3) = – 2𝜋/3
Which does lie in range − 𝜋 < 𝜃 ≤ 𝜋.

Thus Argument of Complex Number z = – 1 – i √3 is – 2𝜋/3


2. z = – √3 + i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – √3 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = – √3
rSinθ = 1

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (- √3)2 + (1)2 = 3 + 1 = 4

⇒ r2Cos2θ + r2Sin2θ = 4

r2(Cos2θ + Sin2θ) = 4

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 4

r2 = 4

r = ± 2
Thus r = 2 or -2 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = 2

Hence Modulus of Complex Number z = – √3 + i is 2


Now let’s find out Argument of the Complex Number z = – √3 + i
Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – √3 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = – √3
rSinθ = 1

Replacing r = 2 in these equations
2Cosθ = – √3
2Sinθ = 1

⇒ Cosθ = – √3/2
⇒ Sinθ = 1/2

⇒ θ = 𝜋/6

As Cosine is negative but Sine is positive thus θ must lie in the second quadrant and as θ = 𝜋/6 lie in range − 𝜋 < 𝜃 ≤ 𝜋. Thus it is the argument of given Complex Number z = – √3 + i.

Thus Argument of Complex Number z = – √3 + i is 𝜋/6


Convert each of the complex numbers given in Exercises 3 to 8 in the polar form:

3. 1 – i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = 1 – i with z = rCosθ + i(rSinθ)
We get
rCosθ = 1
rSinθ = – 1

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (1)2 + (- 1)2 = 1 + 1 = 2

⇒ r2Cos2θ + r2Sin2θ = 2

r2(Cos2θ + Sin2θ) = 2

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 2

r2 = 2

r = ± √2
Thus r = √2 or – √2 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = √2

Hence Modulus of Complex Number z = 1 – i is r = √2


Now let’s find out Argument of the Complex Number z = 1 – i
Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = 1 – i with z = rCosθ + i(rSinθ)
We get
rCosθ = 1
rSinθ = – 1

Replacing r = √2 in these equations
√2Cosθ = 1
√2Sinθ = – 1

⇒ Cosθ = 1/√2
⇒ Sinθ = – 1/√2

As Sine is negative but Cosine is positive thus θ must lie in fourth quadrant
So for Cosθ = 1/√2 and Sinθ = – 1/√2 value of θ = 7𝜋/4

But by definition argument of a Complex Number θ must lie in range − 𝜋 < 𝜃 ≤ 𝜋
And as θ = 7𝜋/4 lies in fourth quadrant thus it can also be written as
– (2𝜋 – 7𝜋/4) = – 𝜋/4

Thus θ = – 𝜋/4 which does lie in range − 𝜋 < 𝜃 ≤ 𝜋 and is thus Argument of Complex Number z = 1 – i

So
r = √2
θ = – 𝜋/4

Polar Form = rCosθ + i(rSinθ) = √2Cos(- 𝜋/4) + i(√2Sin(- 𝜋/4))
= √2[Cos(- 𝜋/4) + i Sin(- 𝜋/4)]

Thus Polar Form of Complex Number z = 1 – i is √2[Cos(- 𝜋/4) + i Sin(- 𝜋/4)]


4. – 1 + i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = 1

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (-1)2 + (1)2 = 1 + 1 = 2

⇒ r2Cos2θ + r2Sin2θ = 2

r2(Cos2θ + Sin2θ) = 2

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 2

r2 = 2

r = ± √2
Thus r = √2 or – √2 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = √2

Hence Modulus of Complex Number z = – 1 + i is √2


Now let’s find out Argument of the Complex Number z = – 1 + i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = 1

Replacing r = √2 in these equations
√2Cosθ = – 1
√2Sinθ = 1

⇒ Cosθ = – 1/√2
⇒ Sinθ = 1/√2

As Cosine is negative but Sine is positive thus θ must lie in the second quadrant
⇒ θ = 3𝜋/4

And by definition Argument of any Complex Number θ must lie in range − 𝜋 < 𝜃 ≤ 𝜋 which θ = 3𝜋/4 does lies in.

Hence it’s the argument of given Complex Number z = – 1 + i

Thus
r = √2
θ = 3𝜋/4

Polar Form of Complex Number = rCosθ + i(rSinθ)
Replacing r and θ

⇒ Polar Form of Complex Number – 1 + i = √2Cos(3𝜋/4) + i[√2Sin(3𝜋/4)]
= √2[Cos(3𝜋/4) + iSin(3𝜋/4)]

Thus Polar Form of Complex Number z = – 1 + i is √2[Cos(3𝜋/4) + iSin(3𝜋/4)]


5. – 1 – i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 – i with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = – 1

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (- 1)2 + (- 1)2 = 1 + 1 = 2

⇒ r2Cos2θ + r2Sin2θ = 2

r2(Cos2θ + Sin2θ) = 2

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 2

r2 = 2

r = ± √2
Thus r = √2 or – √2 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = √2

Hence Modulus of Complex Number z = – 1 – i is √2


Now let’s find out Argument of the Complex Number z = – 1 – i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 – i with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = – 1

Replacing r = √2 in these equations
√2Cosθ = – 1
√2Sinθ = – 1

⇒ Cosθ = – 1/√2
⇒ Sinθ = – 1/√2

As Cosine is negative but Sine is positive thus θ must lie in the third quadrant
⇒ θ = 5𝜋/4

And by definition Argument of any Complex Number θ must lie in range − 𝜋 < 𝜃 ≤ 𝜋 which θ = 5𝜋/4 does not lies in.

Thus measuring θ = 5𝜋/4 from fourth quadrant in the reverse direction we get
– (2𝜋 – 5𝜋/4) = – 3𝜋/4 which does lies in range − 𝜋 < 𝜃 ≤ 𝜋

Hence Argument θ of Complex Number – 1 – i is -3𝜋/4

Thus
r = √2
θ = – 3𝜋/4

Polar Form of Complex Number = rCosθ + i(rSinθ)
Replacing r and θ

⇒ Polar Form of Complex Number – 1 – i = √2Cos(- 3𝜋/4) + i[√2Sin(- 3𝜋/4)]
= √2[Cos(- 3𝜋/4) + iSin( 3𝜋/4)]

Thus Polar Form of Complex Number z = – 1 – i is √2[Cos(- 3𝜋/4) + iSin(- 3𝜋/4)]


6. – 3

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 3 with z = rCosθ + i(rSinθ)
We get
rCosθ = – 3
rSinθ = 0

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (- 3)2 + (0)2 = 9 + 0 = 9

⇒ r2Cos2θ + r2Sin2θ = 9

r2(Cos2θ + Sin2θ) = 9

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 9

r2 = 9

r = ± 3
Thus r = 3 or 3 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = 3

Hence Modulus of Complex Number z = – 3 is 3


Now let’s find out Argument of the Complex Number z = – 3

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 3 with z = rCosθ + i(rSinθ)
We get
rCosθ = – 3
rSinθ = 0

Replacing r = 3 in these equations
3Cosθ = – 3
3Sinθ = 0

⇒ Cosθ = – 3/3 = – 1
⇒ Sinθ = 0

As Cosine is negative but Sine is positive thus θ must lie in the second quadrant
⇒ θ = 𝜋

And by definition Argument of any Complex Number θ must lie in range − 𝜋 < 𝜃 ≤ 𝜋 which θ = 𝜋 does lies in.

Thus θ = 𝜋 is the Argument of given Complex Number z = – 3

Thus
r = 3
θ = 𝜋

Polar Form of Complex Number = rCosθ + i(rSinθ)
⇒ 3Cos𝜋 + i(3Sin𝜋)
= 3[Cos𝜋 + i Sin𝜋]

Hence Polar Form of Complex Number z = – 3 is 3[Cos𝜋 + i Sin𝜋]


7. √3 + i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = √3 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = √3
rSinθ = 1

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (√3)2 + (1)2 = 3 + 1 = 4

⇒ r2Cos2θ + r2Sin2θ = 4

r2(Cos2θ + Sin2θ) = 4

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 4

r2 = 4

r = ± 2
Thus r = 2 or – 2 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = 2

Hence Modulus of Complex Number z = √3 + i is 2


Now let’s find out Argument of the Complex Number z = √3 + i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = √3 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = √3
rSinθ = 1

Replacing r = 2 in these equations
2Cosθ = √3
2Sinθ = 1

⇒ Cosθ = √3/2
⇒ Sinθ = 1/2

As both of Sine and Cosine are positive thus θ must lie in first quadrant
⇒ θ = 𝜋/6

And by definition Argument of any Complex Number θ must lie in range − 𝜋 < 𝜃 ≤ 𝜋 which θ = 𝜋/6 does lies in.

Hence Argument of Complex Number z = √3 + i is θ = 𝜋/6

Thus
r = 2
θ = 𝜋/6

Polar Form of Complex Number = rCosθ + i(rSinθ)
= 2Cos(𝜋/6) + i(2Sin(𝜋/6))
= 2[Cos(𝜋/6) + i Sin(𝜋/6)]

Thus Polar form of Complex Number z = √3 + i is 2[Cos(𝜋/6) + i Sin(𝜋/6)]


8. i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = i with z = rCosθ + i(rSinθ)
We get
rCosθ = 0
rSinθ = i

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (0)2 + (1)2 = 0 + 1 = 1

⇒ r2Cos2θ + r2Sin2θ = 1

r2(Cos2θ + Sin2θ) = 1

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 1

r2 = 1

r = ± 1
Thus r = 1 or – 1 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = 1

Hence Modulus of Complex Number z = i is 1


Now let’s find out Argument of the Complex Number z = i

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = i with z = rCosθ + i(rSinθ)
We get
rCosθ = 0
rSinθ = 1

Replacing r = 1 in these equations
1 × Cosθ = 0
1 × Sinθ = 1

⇒ Cosθ = 0
⇒ Sinθ = 1

As both of Sine and Cosine are positive thus θ must lie in first quadrant
⇒ θ = 𝜋/2

And by definition Argument of any Complex Number θ must lie in range − 𝜋 < 𝜃 ≤ 𝜋 which θ = 𝜋/2 does lies in.

Hence Argument of Complex Number z = i is θ = 𝜋/2

Thus
r = 1
θ = 𝜋/2

Polar Form of Complex Number = rCosθ + i(rSinθ)
= 1 × Cos(𝜋/2) + i(1 × Sin(𝜋/2))
= Cos(𝜋/2) + i Sin(𝜋/2)

Thus Polar form of Complex Number z = i is Cos(𝜋/2) + i Sin(𝜋/2)


Exercise 5.3

Solve each of the following equations:

1. x2 + 3 = 0

x2 + 3 = 0

x2 = – 3

x = ± √-3

This can be rewritten as following
x = ± √3 × √-1

Replacing √-1 with Imaginary Number iota (i)

⇒ x = ± √3i

Thus value of x is ± √3i if its given that x2 + 3 = 0


2. 2x2 + x + 1 = 0

Comparing 2x2 + x + 1 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = 2
⇒ b = 1
⇒ c = 1

Finding out Discriminant of quadratic equation 2x2 + x + 1 = 0
D = b2 – 4ac

D = (1)2 – 4 × 2 × 1
D = 1 – 8

D = -7

Finding out solution of Quadratic Equation 2x^2 + x + 1 = 0

3. x2 + 3x + 9 = 0

Comparing x2 + 3x + 9 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = 1
⇒ b = 3
⇒ c = 9

Finding out Discriminant of quadratic equation x2 + 3x + 9 = 0
D = b2 – 4ac

D = (3)2 – 4 × 1 × 9
D = 9 – 36

D = -27

Finding out solution of Quadratic Equation x^2 + 3x + 9 = 0

4. – x2 + x – 2 = 0

Comparing – x2 + x – 2 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = – 1
⇒ b = 1
⇒ c = – 2

Finding out Discriminant of quadratic equation – x2 + x – 2 = 0
D = b2 – 4ac

D = (1)2 – 4 × (- 1) × (- 2)
D = 1 – 8

D = – 7

Finding out solution of Quadratic Equation - x^2 + x - 2 = 0

5. x2 + 3x + 5 = 0

Comparing x2 + 3x + 5 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = 1
⇒ b = 3
⇒ c = 5

Finding out Discriminant of quadratic equation x2 + 3x + 5 = 0
D = b2 – 4ac

D = (3)2 – 4 × 1 × 5
D = 9 – 20 = – 11

D = – 11

Finding out solution of Quadratic Equation x^2 + 3x + 5 = 0

6. x2 – x + 2 = 0

Comparing x2 – x + 2 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = 1
⇒ b = – 1
⇒ c = 2

Finding out Discriminant of quadratic equation x2 – x + 2 = 0
D = b2 – 4ac

D = (- 1)2 – 4 × 1 × 2
D = 1 – 8 = – 7

D = – 7

Finding out solution of Quadratic Equation x^2 - x + 2 = 0

8. √3x2 – √2x + 3√3 = 0

Comparing √3x2 – √2x + 3√3 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = √3
⇒ b = – √2
⇒ c = 3√3

Finding out Discriminant of quadratic equation √3x2 – √2x + 3√3 = 0
D = b2 – 4ac

D = (√3)2 – 4 × √3 × 3√3
D = 3 – 36 = – 33

D = – 33

Finding out solution of Quadratic Equation √3x^2 - √2x + 3√3 = 0

9. x2 + x + 1/√2 = 0

Simplifying Quadratic Equation x2 + x + 1/√2 = 0

√2x2 + √2x + 1 = 0

Let’s find out which values of x satisfy this Quadratic Equation

Comparing √2x2 + √2x + 1 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = √2
⇒ b = √2
⇒ c = 1

Finding out Discriminant of quadratic equation √2x2 + √2x + 1 = 0
D = b2 – 4ac

D = (√2)2 – 4 × √2 × 1
D = 2 – 4√2 = 2(1 – 2√2)

D = – 2(2√2 – 1)

Finding out solution of Quadratic Equation √2x^2 + √2x + 1 = 0

10. x2 + x/√2 + 1 = 0

Let’s first simplify this Quadratic Equation and then find out values of variable x.

x2 + x/√2 + 1 = 0 can be rewritten as √2x2 + x + √2 = 0

Let’s now find out values of x for which Quadratic Equation √2x2 + x + √2 = 0 is satisfied

Comparing √2x2 + x + √2 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = √2
⇒ b = 1
⇒ c = √2

Finding out Discriminant of quadratic equation √2x2 + √2x + 1 = 0
D = b2 – 4ac

D = (1)2 – 4 × √2 × √2
D = 1 – 4 × 2 = 1 – 8 = – 7

D = – 7

Finding out solution of Quadratic Equation √2x^2 + √2x + 1 = 0

Miscellaneous Exercise

1. Evaluate [i18 + (1/i)25]3

Let’s simplify this equation

\begin{equation} \left[ i^{18} + \left( \frac{1}{i} \right)^{25} \right]^{3} \\ \text{ } \\ \left[ (i^{4})^{4}i^{2} + \left( \frac{1}{i} \right)^{25} \right]^{3} \\ \text{ } \\ \left[ (i^{4})^{4}i^{2} + \frac{1}{i^{25}} \right]^{3} \\ \text{ } \\ \left[ (i^{4})^{4}i^{2} + \frac{1}{(i^{4})^{6}i} \right]^{3} \\ \text{ } \\ \text{Replacing } i^{4} = 1 \\ \text{ } \\ \left[ (1)^{4}i^{2} + \frac{1}{(1)^{6}i} \right]^{3} \\ \text{ } \\ \left[ i^{2} + \frac{1}{i} \right]^{3} \\ \text{ } \\ \text{Replacing } i^{2} = – 1 \\ \text{ } \\ \left[ – 1 + \frac{1}{i} \right]^{3} \\ \text{ } \\ \text{Replacing } \frac{1}{i} = – i \\ \text{ } \\ \left[ – 1 + (- i) \right]^{3} \\ \text{ } \\ \left[ (-1)(1 + i) \right]^{3} \\ \text{ } \\ (-1)^{3}(1 + i)^{3} \\ \text{ } \\ – (1 + i)^{3} \\ \text{ } \\ \text{Using formula } \\ \text{ } \\ (a + b)^{3} = a^{3} + b^{3} + 3ab(a + b) \\ \text{ } \\ – \left[ 1^{3} + i^{3} + 3 × 1 × i (1 + i) \right] \\ \text{ } \\ – \left[ 1 + i^{3} + 3i(1 + i) \right] \\ \text{ } \\ – \left[ 1 + i^{3} + 3i + 3i^{2} \right] \\ \text{ } \\ \text{Replacing } i^{3} = – i \\ \text{ } \\ – \left[ 1 + (- i) + 3i + 3i^{2} \right] \\ \text{ } \\ \text{Replacing } i^{2} = – 1 \\ \text{ } \\ – \left[ 1 + (- i) + 3i + 3(- 1) \right] \\ \text{ } \\ – \left[ 1 – i + 3i – 3 \right] \\ \text{ } \\ – \left[ – 2 + 2i \right] \\ \text{ } \\ 2 – 2i \end{equation}

2. For any two complex numbers z1 and z2, prove that Re(z1 z2) = Re(z1) Re(z2) – Im(z1) Im(z2)

Let’s suppose that
z1 = a + ib
z2 = c + id

Where a, b, c, d are Real Numbers and i is Imaginary Number iota

z1 z2 = (a + ib)(c + id) = a(c + id) + ib(c + id)

z1 z2 = ac + iad + ibc + i2bd

Replacing i2 = – 1

z1 z2 = ac + iad + ibc + (- 1)bd

z1 z2 = (ac – bd) + i(ad + bc)

⇒ Re(z1 z2) = ac – bd [Equation 1]

We have already supposed that z1 = a + ib and z2 = c + id
Re(z1) = a
Re(z2) = c

Im(z1) = b
Im(z2) = d

Replacing these values in [Equation 1]
⇒ Re(z1 z2) = Re(z1) Re(z2) – Im(z1) Im(z2)

Hence if z1 and z2 are two Complex Numbers then Re(z1 z2) = Re(z1) Re(z2) – Im(z1) Im(z2)


Simplifying Complex Number Equation (1/1 - 4i - 2/1 + i)(3- 4i/5 + i)

4. If x – iy = √a – ib/c – id prove that (x2 + y2)2 = a2 + b2/c2 + d2


5. Convert the following in the polar form:

1. 1 + 7i/(2 – i)2

Converting – 1 + i into Polar Form

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = 1

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (- 1)2 + (1)2 = 1 + 1 = 2

⇒ r2Cos2θ + r2Sin2θ = 2

r2(Cos2θ + Sin2θ) = 2

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 2

r2 = 2

r = ± √2
Thus r = √2 or – √2 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = √2

Hence Modulus of Complex Number z = – 1 + i is r = √2


Now let’s find out Argument of the Complex Number z = – 1 + i
Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = 1

Replacing r = √2 in these equations
√2Cosθ = – 1
√2Sinθ = 1

⇒ Cosθ = – 1/√2
⇒ Sinθ = 1/√2

As Sine is positive but Cosine is negative thus θ must lie in Second Quadrant
So for Cosθ = – 1/√2 and Sinθ = 1/√2 value of θ = 3𝜋/4

By definition argument of a Complex Number θ must lie in range − 𝜋 < 𝜃 ≤ 𝜋 and θ = 3𝜋/4 does lies in this interval hence θ = 3𝜋/4 is argument of given complex number – 1 + i

So
r = √2
θ = 3𝜋/4

Polar Form = rCosθ + i(rSinθ) = √2Cos(3𝜋/4) + i(√2Sin(3𝜋/4))
= √2[Cos(3𝜋/4) + i Sin(3𝜋/4)]

Thus Polar Form of Complex Number z = – 1 + i is √2[Cos(3𝜋/4) + i Sin(3𝜋/4)]

Hence Polar Form of equation (1 + 7i)/(2 – i)2 = – 1 + i is √2[Cos(3𝜋/4) + i Sin(3𝜋/4)]


2. (1 + 3i)/(1 – 2i)

Simplifying Complex Number equation (1 + 3i)/(1 -2i)

Converting – 1 + i into Polar Form

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = 1

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (- 1)2 + (1)2 = 1 + 1 = 2

⇒ r2Cos2θ + r2Sin2θ = 2

r2(Cos2θ + Sin2θ) = 2

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 2

r2 = 2

r = ± √2
Thus r = √2 or – √2 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = √2

Hence Modulus of Complex Number z = – 1 + i is r = √2


Now let’s find out Argument of the Complex Number z = – 1 + i
Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1 + i with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1
rSinθ = 1

Replacing r = √2 in these equations
√2Cosθ = – 1
√2Sinθ = 1

⇒ Cosθ = – 1/√2
⇒ Sinθ = 1/√2

As Sine is positive but Cosine is negative thus θ must lie in Second Quadrant
So for Cosθ = – 1/√2 and Sinθ = 1/√2 value of θ = 3𝜋/4

By definition argument of a Complex Number θ must lie in range − 𝜋 < 𝜃 ≤ 𝜋 and θ = 3𝜋/4 does lies in this interval hence θ = 3𝜋/4 is argument of given complex number – 1 + i

So
r = √2
θ = 3𝜋/4

Polar Form = rCosθ + i(rSinθ) = √2Cos(3𝜋/4) + i(√2Sin(3𝜋/4))
= √2[Cos(3𝜋/4) + i Sin(3𝜋/4)]

Thus Polar Form of Complex Number z = – 1 + i is √2[Cos(3𝜋/4) + i Sin(3𝜋/4)]

Hence Polar Form of equation (1 + 3i)/(1 – 2i) = – 1 + i is √2[Cos(3𝜋/4) + i Sin(3𝜋/4)]


Solve each of the equations in Exercises 6 to 9.

6. 3x2 – 4x + 20/3 = 0

Comparing 9x2 – 12x + 20 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = 9
⇒ b = – 12
⇒ c = 20

Finding out Discriminant of quadratic equation 2x2 + x + 1 = 0
D = b2 – 4ac

D = (- 12)2 – 4 × 9 × 20
D = 144 – 720

D = – 576

Finding out roots of Quadratic Equation 9x^2 - 12x + 20 = 0

7. x2 – 2x + 3/2 = 0

This Quadratic Equation can be rewritten as 2x2 – 4x + 3 = 0

Comparing 2x2 – 4x + 3 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = 2
⇒ b = – 4
⇒ c = 3

Finding out Discriminant of quadratic equation 2x2 – 4x + 3 = 0
D = b2 – 4ac

D = (- 4)2 – 4 × 2 × 3
D = 16 – 24

D = – 8

Finding roots of Quadratic Equation 2x^2 - 4x + 3 = 0

8. 27x2 – 10x + 1 = 0

Comparing 27x2 – 10x + 1 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = 27
⇒ b = – 10
⇒ c = 1

Finding out Discriminant of quadratic equation 27x2 – 10x + 1 = 0
D = b2 – 4ac

D = (- 10)2 – 4 × 27 × 1
D = 100 – 108

D = – 8

Finding Roots of Quadratic Equation 27x^2 - 10x + 1 = 0

9. 21x2 – 28x + 10 = 0

Comparing 21x2 – 28x + 10 = 0 with standard quadratic equation ax2 + bx + c = 0
⇒ a = 21
⇒ b = – 28
⇒ c = 10

Finding out Discriminant of quadratic equation 27x2 – 10x + 1 = 0
D = b2 – 4ac

D = (- 28)2 – 4 × 21 × 10
D = 784 – 840

D = – 56

Finding Roots of Quadratic Equation 21x^2 - 28x + 10 = 0

10. If z1 = 2 – i, z2 = 1 + i then find |z1 + z2 + 1/z1 – z2 + 1|

Finding Modulus of |(z1 + z2 + 1)/(z1 - z2 + 1)| if z1 = 2 - i and z2 = 1 + i

Hence if z1 = 2 – i and z2 = 1 + i then |z1 + z2 + 1/z1 – z2 + 1| = √2


11. If a + ib = (x + i)2/2x2 + 1 then prove that a2 + b2 = (x2 + 1)2/(2x2 + 1)2

If a + ib = (x + i)^2/2x^2 + 1 then prove that a^2 + b^2 = (x^2 + 1)^2/(2x^2 + 1)^2

12. Let z1 = 2 – i and z2 = – 2 + i then Find

\begin{equation} \text { (i) } \operatorname{Re}\left(\frac{z_{1} z_{2}}{\bar{z}_{1}}\right) \\ \text{ } \\ \text{Re} \left( \frac { (2 – i)(- 2 + i) } { \overline{(2 – i)} } \right) \\ \text{ } \\ \text{Re} \left( \frac { 2(- 2 + i) – i(- 2 + i) } { \overline{(2 – i)} } \right) \\ \text{ } \\ \text{Re} \left( \frac { – 4 + 2i + 2i – i^{2} } { 2 + i } \right) \\ \text{ } \\ \text{Re} \left( \frac { – 4 + 4i – i^{2} } { 2 + i } \right) \\ \text{ } \\ \text{Replacing } i^{2} = – 1 \\ \text{ } \\ \text{Re} \left( \frac { – 4 + 4i – (- 1) } { 2 + i } \right) \\ \text{ } \\ \text{Re} \left( \frac { – 4 + 4i + 1 } { 2 + i } \right) \\ \text{ } \\ \text{Re} \left( \frac { – 3 + 4i } { 2 + i } \right) \\ \text{ } \\ \text{Multiplying and Dividing by (2 – i)} \\ \text{ } \\ \text{Re} \left( \frac { (- 3 + 4i)(2 – i) } { (2 + i)(2 – i) } \right) \\ \text{ } \\ \text{Re} \left( \frac { – 3(2 – i) + 4i(2 – i) } { (2 + i)(2 – i) } \right) \\ \text{ } \\ \text{Using formula } (a + b)(a – b) = a^{2} – b^{2} \\ \text{ } \\ \text{Re} \left( \frac { – 3(2 – i) + 4i(2 – i) } { 2^{2} – i^{2} } \right) \\ \text{ } \\ \text{Replacing } i^{2} = – 1 \\ \text{ } \\ \text{Re} \left( \frac { – 3(2 – i) + 4i(2 – i) } { 2^{2} – (- 1) } \right) \\ \text{ } \\ \text{Re} \left( \frac { – 6 + 3i + 8i – 4i^{2} } { 4 + 1 } \right) \\ \text{ } \\ \text{Re} \left( \frac { – 6 + 3i + 8i – 4i^{2} } { 5 } \right) \\ \text{ } \\ \text{Replacing } i^{2} = – 1 \\ \text{ } \\ \text{Re} \left( \frac { – 6 + 3i + 8i – 4(- 1) } { 5 } \right) \\ \text{ } \\ \text{Re} \left( \frac { – 6 + 3i + 8i + 4 } { 5 } \right) \\ \text{ } \\ \text{Re} \left( \frac { – 2 + 11i } { 5 } \right) \\ \text{ } \\ \text{Re} \left( \frac{- 2}{5} + \frac{11i}{5} \right) \\ \text{ } \\ \implies \frac{- 2}{5} \end{equation}


13. Find the modulus and argument of the complex number 1 + 2i/1 – 3i

Let’s first simplify this complex number and then find out it’s modulus and argument

\begin{equation} \frac{1 + 2i}{1 – 3i} \\ \text{ } \\ \text{Multiplying and Dividing by } (1 + 3i) \\ \text{ } \\ \frac{(1 + 2i)(1 + 3i)}{(1 – 3i)(1 + 3i)} \\ \text{ } \\ \frac{1(1 + 3i) + 2i(1 + 3i)}{(1 – 3i)(1 + 3i)} \\ \text{ } \\ \frac{1 + 3i + 2i + 6i^{2}}{(1 – 3i)(1 + 3i)} \\ \text{ } \\ \text{Using formula } \\ (a – b)(a + b) = a^{2} – b^{2} \\ \text{ } \\ \frac{1 + 5i + 6i^{2}}{1^{2} – (3i)^{2}} \\ \text{ } \\ \frac{1 + 5i + 6i^{2}}{1 – 3^{2}i^{2}} \\ \text{ } \\ \frac{1 + 5i + 6i^{2}}{1 – 9i^{2}} \\ \text{ } \\ \text{Replacing } i^{2} = – 1 \\ \text{ } \\ \frac{1 + 5i + 6(- 1)}{1 – 9(- 1)} \\ \text{ } \\ \frac{1 + 5i – 6}{1 + 9} \\ \text{ } \\ \frac{- 5 + 5i}{10} \\ \text{ } \\ \frac{- 5}{10} + \frac{5i}{10} \\ \text{ } \\ \frac{- 1}{2} + \frac{i}{2} \\ \text{ } \\ \end{equation}

So we have simplified Complex Number given in the question now let’s move onto finding out Modulus and Argument of this simplified complex number.

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1/2 + i/2 with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1/2
rSinθ = 1/2

Squaring both of these equations and then adding
r2Cos2θ + r2Sin2θ = (-1/2)2 + (1/2)2 = 1/4 + 1/4 = 2/4 = 1/2

⇒ r2Cos2θ + r2Sin2θ = 1/2

r2(Cos2θ + Sin2θ) = 1/2

Sine square plus Cos square is always equal to 1 for any value of angle
⇒ r2(1) = 1/2

r2 = 1/2

r = ± 1/√2
Thus r = 1/√2 or – 1/√2 but by definition of Modulus of a Complex Number it cannot be negative thus only possible value of r = 1/√2

Hence Modulus of Complex Number z = – 1/2 + i/2 is 1/√2


Now let’s find out Argument of the Complex Number z = – 1/2 + i/2

Any Complex Number of form z = a + ib can be written as z = rCosθ + i(rSinθ) where
rCosθ = a
rSinθ = b

On Comparing z = – 1/2 + i/2 with z = rCosθ + i(rSinθ)
We get
rCosθ = – 1/2
rSinθ = 1/2

Replacing r = 2 in these equations
√2Cosθ = – 1
√2Sinθ = 1

⇒ Cosθ = – 1/√2
⇒ Sinθ = 1/√2

As Cosine is negative and Sine is positive thus θ must lie in Second quadrant
⇒ θ = 3𝜋/4

By definition Argument of a Complex Number θ lie in range − 𝜋 < 𝜃 ≤ 𝜋 and θ = 3𝜋/4 does lie in this range hence it’s argument of the complex number z = – 1/2 + i/2

Thus Modulus and Argument of Complex Number (1 + 2i)/(1 – 3i) is √2 and 3𝜋/4


14. Find the real numbers x and y if (x – iy)(3 + 5i) is the conjugate of – 6 – 24i

Let’s first simplify (x – iy)(3 + 5i) and rewrite it in usual form of a complex number z = a + ib
(x – iy)(3 + 5i) = x(3 + 5i) – iy(3 + 5i)

= 3x + i(5x) – i(3y) – i2(5y)

Replacing i2 = – 1

= 3x + i(5x) – i(3y) – (- 1)(5y)

= 3x + i(5x) – i(3y) + 5y

= (3x + 5y) + i(5x – 3y)

(x – iy)(3 + 5i) = (3x + 5y) + i(5x – 3y)

As per question it’s given that – 6 – 24i is conjugate of (x – iy)(3 + 5i)

By definition of Conjugate of a complex number z = a + ib we know that it’s same complex number except sign for imaginary part is flipped
⇒ z = a + ib
⇒ Conjugate of z = a – ib

Using this logic but for values given in the question
⇒ z = – 6 – 24i
⇒ Conjugate of z = – 6 + 24i

As per question
⇒ Conjugate of z = (x – iy)(3 + 5i) = (3x + 5y) + i(5x – 3y)

⇒ Conjugate of z = – 6 + 24i = (3x + 5y) + i(5x – 3y)

– 6 + 24i = (3x + 5y) + i(5x – 3y)
Comparing
Real and Imaginary parts on both sides of this equation
We get

⇒ 3x + 5y = – 6
⇒ 5x – 3y = 24

Simplifying 3x + 5y = - 6 and 5x - 3y = 24 to find out values of x and y

15. Find the modulus of (1 + i)/(1 – i) – (1 – i)/(1 + i)

Find the modulus of (1 + i)/(1 - i) - (1 - i)/(1 + i) (part 1)
Find the modulus of (1 + i)/(1 - i) - (1 - i)/(1 + i) (part 2)

16. If (x + iy)3 = u + iv then show that u/x + v/y = 4(x2 – y2)

Let’s simplify (x + iy)3 = u + iv

Using formula (a + b)3 = a3 + b3 + 3ab(a + b)

(x + iy)3 = x3 + (iy)3 + 3 × x × iy(x + iy) = u + iv

⇒ x3 + i3y3 + i(3x2y) + i2(3xy2) = u + iv

Replacing
i2 = – 1
i3 = – i

⇒ x3 – iy3 + i(3x2y) – (3xy2) = u + iv

(x3 – 3xy2) + i(3x2y – y3) = u + iv

Comparing Real and Imaginary Parts on both sides of this equation
x3 – 3xy2 = u
3x2y – y3 = v

Simplifying
x(x2 – 3y2) = u
y(3x2 – y2) = v

⇒ x2 – 3y2 = u/x
⇒ 3x2 – y2 = v/y

Adding both of these equations
x2 – 3y2 + 3x2 – y2 = u/x + v/y

4(x2 – y2) = u/x + v/y

Hence if (x + iy)3 = u + iv then u/x + v/y = 4(x2 – y2)


Miscellaneous Exercises Complex Numbers and Quadratic Equations – NCERT Class 11 Mathematics Chapter 5 (question number 17)

18. Find the number of non-zero integral solutions of the equation |1 – i|x = 2x

Taking log2 of both sides of equation |1 – i|x = 2x

⇒ log2(|1 – i|x) = log22x

x log2(|1 – i|) = x log22

Value of log22 = 1

x log2(|1 – i|) = x(1) = x

x log2(|1 – i|) = x

|1 – i| = Square Root of 12 + 12 = √2

Replacing this value in above equation
x log2(√2) = x

x log2(21/2) = x

x (1/2) log22 = x

log22 = 1

⇒ x/2 = x

x = 2x

0 = 2x – x

0 = x

Thus number of non-zero integral solutions of the equation |1 – i|x = 2x is zero


If (a + ib)(c + id)(e + if)(g + ih) = A + iB then show that (a^2 + b^2)(c^2 + d^2)(e^2 + f^2)(g^2 + h^2) = A^2 + B^2

if ((1 + i)/(1 - i))^m = 1 then the least positive integral value of m

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