Question

Convert the following sums or differences into products cos 79 degree + cos 11 degrees?

Answer 1

$$\begin{gathered} \cos 79°+\cos 11° \\ =2\cos \left(\frac{79°+11°}{2}\right).\cos \left(\frac{79°-11°}{2}\right)\left[\mathrm{as},\cos \mathrm{C}+\mathrm{cosD}=2\cos \left(\frac{\mathrm{C}+\mathrm{D}}{2}\right).\cos \left(\frac{\mathrm{C}-\mathrm{D}}{2}\right)\right] \\ =2\cos \left(\frac{90°}{2}\right).\cos \left(\frac{68°}{2}\right) \\ =2\cos 45°.\cos 38° \end{gathered}$$