MHB Ellen's question at Yahoo Answers ( Int 4 (tan ^3 x) dx )

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To evaluate the integral ∫ 4(tan^3 x) dx, the process begins by rewriting it as 4∫(tan^2 x)(tan x) dx. This is further transformed using the identity tan^2 x = sec^2 x - 1, leading to the separation of the integral into two parts: 4∫(sec^2 x)(tan x) dx and -4∫tan x dx. By substituting t = tan x, the first integral simplifies to 2tan^2 x, while the second integral evaluates to -4ln|cos x|. The final result is ∫ 4(tan^3 x) dx = 2tan^2 x + 4ln|cos x| + C.
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Hello Ellen,

We have: $$\begin{aligned}\int 4\tan^3 x\;dx&=4\int (\tan^2x)(\tan x)\:dx\\&=4\int(\sec^2x-1)(\tan x)dx\\&=4\int(\sec^2x)(\tan x)\:dx-4\int\tan x\;dx\end{aligned}$$ If $t=\tan x$, then $dt=\sec^2x\;dx$ so $$\int(\sec^2x)(\tan x)\:dx=\int t\:dt=\frac{t^2}{2}=\frac{\tan^2x}{2}$$ On the other hand:
$$\int\tan x\;dx=\int \frac{\sin x}{\cos x}dx=-\ln |\cos x|$$ As a consequence: $$\boxed{\;\displaystyle\int 4\tan^3 x\;dx=2\tan^2x+4\ln|\cos x|+C\;}$$

P.S. Something must be wrong with Yahoo Answers, when I'm logged in, I can't access to the corresponding page.

Edit: Now, that is all right.
 
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