<s>[INST] Another Basic Integration Question, Possibly By Substitution What's the integral of $f(x)=(1-x^2)^{1/2}$? I tried making $x=\sin(t)$ and doing integration by substitution but I don't think I arrived to the correct answer. All responses are appreciated...
 [/INST] Recall the three most common trigonometric substitutions and how they proceed.
Let $x=\sin\theta$. Then $\,dx = \cos\theta \,d\theta$. And $\theta = \arcsin x$.
$$\begin{align} \int\sqrt{1-x^2}\,dx 
& =\int\sqrt{1 - \sin^2\theta}\,\cos\theta \,d\theta \\ \\ 
& =\int\sqrt{\cos^2\theta}\,\cos\theta \,d\theta \\ \\ 
& = \int\cos^2\theta \,d\theta \\ \\ 
& = \int\frac12(1+\cos2\theta) \,d\theta\\ \\ 
& =\frac12\Big(\theta+\frac{\sin2\theta}2\Big)+C\\ \\ 
& =\frac12\Big(\theta+\sin\theta\cos\theta\Big)+C \\ \\
& = \frac 12 \Big(\arcsin x + x\sqrt{1-x^2}\Big) + C\tag{$\dagger$}\end{align}$$
$(\dagger)$. Since $x = \sin \theta$, $\theta = \arcsin x$. And $\cos x = \sqrt{1 - x^2}$. 
How do we know $\cos x = \sqrt{1-x^2}\;?\;$ Note: $\sin^2 \theta + \cos ^2 \theta = 1 = x^2 + (\sqrt{1 - x^2})^2 = x^2 + 1 - x^2$.
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