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If $x$ is a non-right angle in a right angled triangle
then $\sin \left(x\right)$ is the ratio of the length of the side opposite $x$ with the hypotenuse of the triangle
If we restrict our answer to $x$ within $\left[0 , 2 \pi\right]$
$\sin \left(x\right) = 1$ only occurs when $x = \frac{\pi}{2}$