The addition formulas for sine and cosine look like this:
\begin{aligned}
\cos(\alpha + \beta) &= \cos \alpha \cos \beta – \sin \alpha \sin \beta, \\
\sin(\alpha + \beta) &= \cos \alpha \sin \beta + \sin \alpha \cos \beta. \\
\end{aligned}
\cos(\alpha + \beta) &= \cos \alpha \cos \beta – \sin \alpha \sin \beta, \\
\sin(\alpha + \beta) &= \cos \alpha \sin \beta + \sin \alpha \cos \beta. \\
\end{aligned}
I can never remember them [...]
