7 \(\cos \left ( \alpha \right ) +\cos \left ( \beta \right ) \)

This can be found by adding (3) and (3A). Let

\begin{align} A+B & =\alpha \tag {7}\\ A-B & =\beta \nonumber \end{align}

Then (3)+(3A) now becomes

\begin{align} \cos \left ( \alpha \right ) +\cos \left ( \beta \right ) & =\left ( \cos A\cos B-\sin A\sin B\right ) +\left ( \cos A\cos B+\sin A\sin B\right ) \nonumber \\ & =2\cos A\cos B\tag {9}\end{align}

Now we solve for \(A,B\) from (7). Which gives

\begin{align*} A & =\frac {\alpha +\beta }{2}\\ B & =\frac {\alpha -\beta }{2}\end{align*}

Substituting the above in (9) gives

\[ \cos \left ( \alpha \right ) +\cos \left ( \beta \right ) =2\cos \left ( \frac {\alpha +\beta }{2}\right ) \cos \left ( \frac {\alpha -\beta }{2}\right ) \]