$\sin^{-1} (1 - x) - 2 \sin^{-1} x = \frac{\pi}{2}$, then x is equal to
(a) 0
(b) 1
(c) $\frac{1}{2}$
(d) 2
Choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
$\sin^{-1} (1 - x) - 2 \sin^{-1} x = \frac{\pi}{2}$
Let $x = \sin \theta$
$\theta = \sin^{-1} x$
so $\sin^{-1} (1 - x) = \frac{\pi}{2} + 2 \theta$
$1 - x = \sin (\frac{\pi}{2} + 2 \theta)$
$1 - x = \cos 2 \theta$
$1 - \sin \theta = \cos 2 \theta$
⇒ $1 = \cos 2 \theta + \sin \theta$
$1 = 1 - 2 \sin^2 \theta + \sin \theta$
so $2 \sin^2 \theta = \sin \theta$
$x = \sin \theta = 0, \frac{1}{2}$
at $x = \frac{1}{2}$
LHS ≠ RHS
x = 0
Option: 3