1. ## Trigonometric Identities

I need help trying to determine what number is suppose to be in place for alpha? I do not understand how they came up with Cos = 1/4 in question (d).

Sin(90 deg - alpha) = Cos alpha = 1/4

2. Hello,

$\sin(90°-\alpha)=\cos(\alpha)$ can be checked on a unit circle.

Otherwise, use this formula : $\sin(a-b)=\sin(a)\cos(b)-\sin(b)\cos(a)$

So here $\sin(90°-\alpha)={\color{red}\sin(90°)}\cos(\alpha)-\sin(\alpha) {\color{red}\cos(90°)}$

The red parts are supposed to be known