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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 } $

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$$\sin { y } =\cos { \left( y-\frac { \pi }{ 2 } \right) } \\$$ or $$ \sin { y=\sqrt { 1-\cos ^{ 2 }{ y } } } $$

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Recall the Pythagorean identity:

$$a^2+b^2=c^2$$

Divide both sides by $c^2$:

$$\sin^2(\theta)+\cos^2(\theta)=1$$

Solve for $\sin$.

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