Question

What are the formulae for $\cos (c+d)$ and $\cos (c-d)$.

Answer 1

Find the formula for $\cos (c+d)$and $\cos (c-d)$.

$\cos (c+d)$ and $\cos (c-d)$ are the important trigonometric identities also called cosine addition and subtraction formula in the trigonometry.

$$\begin{gathered} \cos (c+d)=\cos c\cos d-\sin c\sin d \\ \cos (c-d)=\cos c\cos d+\sin c\sin d \end{gathered}$$

Here, $c$ and $d$ are the measures of angles.

Hence,

The formula for $\cos (c+d)$ and $\cos (c-d)$ are

$$\begin{gathered} \cos (c+d)=\cos c\cos d-\sin c\sin d \\ \cos (c-d)=\cos c\cos d+\sin c\sin d \end{gathered}$$