Improper integrals, one-sided limits and trigonometric functions 
Evaluate $$\int \limits _{-1}^1\frac{\mathrm d x}{\sqrt{1-x^2}}$$

In my attempt I have 
$$\int \limits _{-1}^0\frac{\mathrm d x}{\sqrt{1-x^2}}+\int \limits _0^1\frac{\mathrm d x}{\sqrt{1-x^2}}=\lim\limits _{a\rightarrow -1^+}-2\sin ^{-1}a +\lim\limits _{b\rightarrow 1^-}2\sin ^{-1}b=\pi +\pi$$
This solution is the same as the one in the textbook I am using.
In the first term, I have $\pi$ when I take $\lim\limits _{a\rightarrow -1^+}-2\sin ^{-1}a=-2\frac{-\pi}{2}$
I don't understand why $\lim\limits _{a\rightarrow -1^+}-2\sin ^{-1}a=-2\frac{3\pi}{2}$ is wrong since $\sin(\frac{3\pi}{2})=-1$
I think that it has something to do with the one sided limit but I really can't figure it out.

Note that the arcsine function is continuous on $[-\pi/2,\pi/2]$ and it is odd, i.e. $f(-x)=-f(x)$. Therefore 
$$\lim_{x\to -1^+}\arcsin(x)=\arcsin(-1)=-\arcsin(1)=-\frac{\pi}{2}.$$
Hence, since $D(\arcsin(x))=\frac{1}{\sqrt{1-x^2}}$ (there is an extra factor $2$ in your attempt), it follows that
$$\int \limits _{-1}^1\frac{1}{\sqrt{1-x^2}}=
\lim_{x\rightarrow 1^-}\arcsin(x)-\lim_{x\rightarrow -1^+}\arcsin(x)=\arcsin(1)-\arcsin(-1)=\frac{\pi}{2}+\frac{\pi}{2}=\pi.$$
