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# If Cos 4 X = 1 + K Sin 2 X Cos 2 X , Then Write the Value of K. - Mathematics

Short Note

If $\cos 4x = 1 + k \sin^2 x \cos^2 x$ , then write the value of k.

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#### Solution

$\text{ We have,}$
$\cos4x = 1 + k \sin^2 x \cos^2 x$
$\Rightarrow \cos\left( 2 \times 2x \right) = 1 + k \sin^2 x \cos^2 x$
$\Rightarrow 1 - 2 \sin^2 2x = 1 + k \sin^2 x \cos^2 x$
$\Rightarrow 1 - 2 \left( 2sinxcosx \right)^2 = 1 + k \sin^2 x \cos^2 x$
$\Rightarrow 1 - 8 \sin^2 x \cos^2 x = 1 + k \sin^2 x \cos^2 x$
$\Rightarrow \sin^2 {xcos}^2 x\left( k + 8 \right) = 0$
$\Rightarrow k + 8 = 0$
$\therefore k = - 8$

Concept: Values of Trigonometric Functions at Multiples and Submultiples of an Angle
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#### APPEARS IN

RD Sharma Class 11 Mathematics Textbook
Chapter 9 Values of Trigonometric function at multiples and submultiples of an angle
Q 1 | Page 42
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