3 and 4 .Determinants and Matrices
hard

माना $f(x)=\left|\begin{array}{ccc}\sin ^{2} x & -2+\cos ^{2} x & \cos 2 x \\ 2+\sin ^{2} x & \cos ^{2} x & \cos 2 x \\ \sin ^{2} x & \cos ^{2} x & 1+\cos 2 x\end{array}\right|, x \in[0, \pi]$है, तो $f ( x )$ का अधिकतम मान बराबर है .............|

A

$6$

B

$7$

C

$8$

D

$9$

(JEE MAIN-2021)

Solution

$\left| {\begin{array}{*{20}{c}} { – 2}&{ – 2}&0 \\ 2&0&{ – 1} \\ {{{\sin }^2}x}&{{{\cos }^2}x}&{1 + \cos 2x} \end{array}} \right| (\mathrm{R}_{1} \rightarrow \mathrm{R}_{1}-\mathrm{R}_{2} \,and \,\mathrm{R}_{2} \rightarrow \mathrm{R}_{2}-\mathrm{R}_{3})$

$-2\left(\cos ^{2} \mathrm{x}\right)+2\left(2+2 \cos 2 \mathrm{x}+\sin ^{2} \mathrm{x}\right)$

$4+4 \cos 2 \mathrm{x}-2\left(\cos ^{2} \mathrm{x}-\sin ^{2} \mathrm{x}\right)$

$\mathrm{f}(\mathrm{x})=4+\underbrace{2 \cos 2 \mathrm{x}}_{\max =1}$

$\mathrm{f}(\mathrm{x})_{\max }=4+2=6$

Standard 12
Mathematics

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