圖
\[\begin{aligned}
\cos(α+β) &= OB \\
& = OD - BD \\
& = OD - EC \\
& = OC \cos \beta - AC \sin \beta \\
& = OA \cos \alpha \cos \beta - OA \sin \alpha \sin \beta \\
& = \cos \alpha \cos \beta - \sin \alpha \sin \beta
\end{aligned}
\]
\[\begin{aligned}
\sin(α+β) &= AB \\
& = AE + BE \\
& = AE + CD \\
& = AC \cos \beta + OC \sin \beta \\
& = OA \sin \alpha \cos \beta + OA \cos \alpha \sin \beta \\
& = \sin \alpha \cos \beta + \cos \alpha \sin \beta \\
\end{aligned}
\]