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JEE Advanced 2009 · Trigonometric Functions

If sin⁡4x2+cos⁡4x3=15\frac{\sin^{4} x}{2} + \frac{\cos^{4} x}{3} = \frac{1}{5}, then

Answers: A. tan⁡2x=23\tan^{2} x = \frac{2}{3} · B. sin⁡8x8+cos⁡8x27=1125\frac{\sin^{8} x}{8} + \frac{\cos^{8} x}{27} = \frac{1}{125}

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If (sin⁴ x)/2 + (cos⁴ x)/3 = 1/5, then sin⁡4x2+cos⁡4x3=15\dfrac{\sin^{4} x}{2} + \dfrac{\cos^{4} x}{3} = \dfrac{1}{5} Find tan⁡2x and sin⁡8x8+cos⁡8x27\text{Find } \tan^{2} x \ \text{and}\ \dfrac{\sin^{8} x}{8} + \dfrac{\cos^{8} x}{27} s=sin⁡2x,c=cos⁡2xs = \sin^{2} x, \quad c = \cos^{2} x s+c=1s + c = 1 s22+c23=15\dfrac{s^{2}}{2} + \dfrac{c^{2}}{3} = \dfrac{1}{5} c=1−sc = 1 - s s22+(1−s)23=15\dfrac{s^{2}}{2} + \dfrac{(1-s)^{2}}{3} = \dfrac{1}{5} 15s2+10(1−s)2=615s^{2} + 10(1-s)^{2} = 6 15s2+10−20s+10s2=615s^{2} + 10 - 20s + 10s^{2} = 6 25s2−20s+4=025s^{2} - 20s + 4 = 0 (5s−2)2=0 ⇒ 5s−2=0 ⇒ s=25(5s - 2)^{2} = 0 \ \Rightarrow \ 5s - 2 = 0 \ \Rightarrow \ s = \dfrac{2}{5} sin⁡2x=25,cos⁡2x=1−25=35\sin^{2} x = \dfrac{2}{5}, \quad \cos^{2} x = 1 - \dfrac{2}{5} = \dfrac{3}{5} tan⁡2x=sin⁡2xcos⁡2x=2/53/5=23\tan^{2} x = \dfrac{\sin^{2} x}{\cos^{2} x} = \dfrac{2/5}{3/5} = \dfrac{2}{3} ⇒option (1) is true, option (3) is false\Rightarrow \text{option (1) is true, option (3) is false} sin⁡8x8+cos⁡8x27=s48+c427\dfrac{\sin^{8} x}{8} + \dfrac{\cos^{8} x}{27} = \dfrac{s^{4}}{8} + \dfrac{c^{4}}{27} s4=(25)4=16625,c4=(35)4=81625s^{4} = \left(\dfrac{2}{5}\right)^{4} = \dfrac{16}{625}, \quad c^{4} = \left(\dfrac{3}{5}\right)^{4} = \dfrac{81}{625} 16/6258+81/62527=2625+3625=5625=1125\dfrac{16/625}{8} + \dfrac{81/625}{27} = \dfrac{2}{625} + \dfrac{3}{625} = \dfrac{5}{625} = \dfrac{1}{125} ⇒option (2) is true, option (4) is false\Rightarrow \text{option (2) is true, option (4) is false} Options (1) and (2) are correct\boxed{\text{Options (1) and (2) are correct}} tan⁡2x=23,sin⁡8x8+cos⁡8x27=1125\tan^{2} x = \dfrac{2}{3}, \quad \dfrac{\sin^{8} x}{8} + \dfrac{\cos^{8} x}{27} = \dfrac{1}{125}

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