Sine of 45 Degrees/Proof 1

Proof

 * Sine45.png

Let $ABCD$ be a square of side $r$.

By definition, each angle of $\triangle ABCD$ is equal to $90^\circ$.

Let $AC$ be a diagonal of $ABCD$.

As $\triangle ABC$ is a right angled triangle, it follows from Pythagoras's Theorem that $AC = \sqrt 2 A B$.

As $AC$ is a bisector of $\angle DAB$ it follows that $\angle CAB = 45^\circ$.

So by definition of sine function:
 * $\sin \left({\angle CAB}\right) = \dfrac r {r \sqrt 2} = \dfrac {\sqrt 2} 2$