78cc822aa6
We reduce computation of `acos` to `asin` as follow:
When `|x| < 0.5`:
```math
acos(x) = \frac{\pi}{2} - asin(x).
```
For `0.5 <= |x| < 1`, let
```math
u = \frac{1 - \left| x \right|}{2},
```
then
```math
acos(x) = \begin{cases}
2 \cdot asin \left( \sqrt{u} \right) &, 0.5 \leq x < 1 \\
\pi - 2 \cdot asin \left( \sqrt{u} \right) &, -1 < x \leq 0.5
\end{cases}
```
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