Question
Find the integral of the function $\tan^3 2x \sec 2x$

Answer

Clearly, $\tan^32x \sec 2x = \tan^22x \tan 2x \sec 2x$
$= (\sec^22x - 1) \tan 2x \sec 2x$
$= \sec^22x.\tan 2x \sec 2x - \tan 2x \sec 2x$
$\Rightarrow \int \tan ^{3} 2 x \sec 2 x d x=\int \sec ^{2} 2 x \tan 2 x \sec 2 x d x-\int \tan 2 x \sec 2 x d x$
$= \int \sec ^{2} 2 x \tan 2 x \sec 2 x d x-\frac{\sec 2 x}{2}+C$
Now, Let sec 2x = t
⇒ 2 sec 2x tan 2x dx = dt
Thus, $\int \tan ^{3} 2 x \sec 2 x d x=\frac{1}{2} \int t^{2} d t-\frac{\sec 2 x}{2}+C$
$= \frac{t^{3}}{6}-\frac{\sec 2 x}{2}+C$
$= \frac{(\sec 2 x)^{3}}{6}-\frac{\sec 2 x}{2}+C$

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