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NCERT Class XI Mathematics - Complex Numbers and Quadratic Equations - Solutions

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Question : 17 of 52
Marks: +1, -0
1 – i
Solution:  
We have, z = 1 – i
Let 1 = r cosθ…(i)
and – 1 = r sinθ …(ii)
Squaring and adding (i) and (ii), we get
r2(cos2θ+sin2θ)r^2(\cos^2 \theta + \sin^2 \theta) = 1 + 1 ⇒ r2r^2 = 2 ⇒ r = 2\sqrt{2}
Substituting the value of r in (i) and (ii), we get 2\sqrt{2} cos θ = 1 , 2\sqrt{2} sin θ = - 1
⇒ cos θ = 12\frac{1}{\sqrt{2}} , sin θ = 12-\frac{1}{\sqrt{2}} ⇒ cos θ = cos π4\frac{\pi}{4} , sin θ = - sin π4\frac{\pi}{4}
Here, cosθ > 0 and sinθ < 0.
∴ θ lies in the fourth quadrant.
∴ θ = π4-\frac{\pi}{4}
∴ The required polar form is
z = 2(cos(π/4)+isin(π/4))\sqrt{2}(\cos(-\pi/4)+i\sin(-\pi/4))
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