Answer
(a) $\arccos(-1)=\pi$
(b) $\arccos0=\frac{\pi}{2}$
Work Step by Step
*Recall the definition of arccosine:
$y=\cos^{-1}x$ is the number in $[0,\pi]$ for which $\cos y=x$
(a) $$\arccos(-1)=a$$
According to the defintion: $$\cos a=-1\hspace{1cm}\text{and}\hspace{1cm}a\in[0,\pi]$$
So $a=\pi$. In other words, $$\arccos(-1)=\pi$$
(b) $$\arccos0=a$$
According to the defintion: $$\cos a=0\hspace{1cm}\text{and}\hspace{1cm}a\in[0,\pi]$$
So $a=\frac{\pi}{2}$. In other words, $$\arccos0=\frac{\pi}{2}$$
