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Q)

# The solution of the equation $4\sin^4x+\cos^4x=1$ is

$\begin{array}{1 1}(a)\;x=2n\pi&(b)\;x=n\pi+1\\(c)\;x=(n+2)\pi&(d)\;None\;of\;these\end{array}$

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A)
Given :
$4\sin ^4x+\cos^4x=1$
$\Rightarrow 4\sin^4x+(\cos^2x+1)(\cos^2x-1)=0$
$4\sin^4x-\sin^2x(1+\cos^2x)=0$
$\sin^2x(5\sin^2x-2)=0$
$\sin^2x=0$
Or $\sin^2x=\large\frac{2}{5}$
$x=n\pi$
$\sin^2x=\large\frac{2}{5}$
$\Rightarrow x=n\pi$
$x=n\pi$
Or $n\pi\pm\sin^{-1}\big(\sqrt{\large\frac{2}{5}}\big)$
Hence (d) is the correct answer.