1.

\( \sin ^{-1} x+\sin ^{-1} 2 x=\frac{\pi}{3} \)

Answer»

sin-1 x +  sin-1 2x = \(\frac{π}{3}\)

sin-1 ( x\((x { \sqrt{1-4x^2} \ } + 2x { \sqrt{1-x^2} \ }) =\frac{π}{3}\)

( ∵ sin-1 x  + sin-1 y = sin-1 \((2x { \ \sqrt{1-y2} \ }+{ 4 \sqrt{1-x^2} \ }))\)\(\)

⇒ \(x { \sqrt{1-4x^2} \ } +2x { \sqrt{1-x^2} \ } =sin\frac{π}{3}=\frac{√3}{2}\)

⇒ x2 (1- 4x2) + 4 x2 (1-x2) + 4x2 \( { \ \sqrt{(1-4x^2)(1-x^2)} \ }=\frac{3}{4}\)

(by squaring on both sides)

⇒ 16x\( { \ \sqrt{(1-x^2)(1-4x^2)} }=3-(x^2 -4x^4+4x^2-4x^4)\)

⇒ \(16x^2 = { \sqrt{(1-x^2)(1-4x^2)} \ } =3-5x^2+8x^4\)

⇒ 256x(1 - x2) (1- 4x2)  = 9 + 25x4+ 64x8 - 30 x2 + 48 x- 80x6

⇒ 256x(1 - 5x+4x) = 64x8 - 80x6 + 73x4 - 30 x2 + 9

⇒ 256x- 1280x + 1024 x = 64 x8 - 80 x6 + 73x4 - 30x2 +9

⇒ 960x8 - 1200x6 + 183x4 +30x2 - 9 = 0



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