How do you find the exact value of sin((3pi)/4)?

$\frac{1}{\sqrt{2}}$
$\sin \left[\frac{3 \pi}{4}\right] = \sin \left[\frac{3 \cdot 180}{4}\right]$= sin 135 degree
sin(90+45) degree = cos 45 degree = $\frac{1}{\sqrt{2}}$