First slide

De-moivre's theorem

Question

If $\cos α + 2 \cos β + 3 \cos γ = \sin α + 2 \sin β + 3 \sin γ = 0$ then the value of sin $3 α + 8 \sin 3 β + 27 \sin 3 γ$ is

Moderate

A

$\sin (α + β + γ)$
B

$3 \sin (α + β + γ)$
C

$18 \sin (α + β + γ)$
D

$\sin (α + 2 β + γ)$

Solution

Let $a = \cos α + isin α$
$\begin{aligned} b = \cos β + isin β \\ c = \cos γ + isin γ \end{aligned}$
Then, $a + 2 b + 3 c = (\cos α + 2 \cos β + 3 \cos γ)$
$+ i (\sin α + 2 \sin β + 3 \sin γ) = 0$
$\begin{array}{l} \Rightarrow a^{3} + 8 b^{3} + 27 c^{3} = 18 abc \\ \Rightarrow \cos 3 α + 8 \cos 3 β + 27 \cos 3 γ = 18 \cos (α + β + γ) \end{array}$
and $\sin 3 α + 8 \sin 3 β + 27 \sin 3 γ = 18 \sin (α + β + γ)$

Get Instant Solutions

When in doubt download our app. Now available Google Play Store- Doubts App

Recieve an sms with download link

Doubts App

OR

SCAN ME

Doubts App

Doubts App