Question

@ghazi Could you please help me with this one question??

Answer 1

Write the expression as either the sine, cosine, or tangent of a single angle.

\[\cos(\pi/5) \cos(\pi/7) + \sin(\pi/5)\sin(\pi/7)\]

Answer 2

Remember that cos(a-b) = cos(a)cos(b) + sin(a)sin(b), for any angles a,b.

If it's true for _any_ angle it must be true for pi/5 and pi/7 too.