Q43 / 120 · VITEEE 2007 · Mathematics

If $\cos x+\cos ^{2} x=1$ then the value of $\sin ^{12} x+3 \sin ^{10} x+3 \sin ^{8} x+\sin ^{6} x-1$ is equal to

  • A
    2
  • B
    1
  • C
    $-1$
  • Correct answer
    D
    0

Solution

Given : $\cos x+\cos ^{2} x=1$ $\begin{array}{l} \Rightarrow \cos x=1-\cos ^{2} x \Rightarrow \cos x=\sin ^{2} x \\ \Rightarrow \cos ^{2} x=\sin ^{4} x \Rightarrow 1-\sin ^{2} x=\sin ^{4} x \\ \Rightarrow \sin ^{4} x+\sin ^{2} x=1 \end{array}$ cubic both sides, we have $\begin{array}{l} \sin ^{12} x+\sin ^{6} x+3 \sin ^{6} x\left(\sin ^{4} x+\sin ^{2} x\right)=1 \\ \Rightarrow \sin ^{12} x+\sin ^{6} x+3 \sin ^{10} x+3 \sin ^{8} x=1 \\ \Rightarrow \sin ^{12} x+3 \sin ^{10} x+3 \sin ^{8} x+\sin ^{6} x-1=0 \end{array}$

Hence, the correct answer is option: 'D'.

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