# How to express sin(y) in terms of cos(y)?

The first question asked to express a equivalent expression in of $\cos(x+y)$ for which I got right.

However its the second part of the question that I do not understand which is How to express $\sin(y)$ in terms of $\cos(y)$? also the angle between $0$ and $\frac { \pi }{ 2 }$

$$\sin { y } =\cos { \left( y-\frac { \pi }{ 2 } \right) } \\$$ or $$\sin { y=\sqrt { 1-\cos ^{ 2 }{ y } } }$$
$$a^2+b^2=c^2$$
Divide both sides by $c^2$:
$$\sin^2(\theta)+\cos^2(\theta)=1$$
Solve for $\sin$.