Question

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

Answer 1

The answer is cos(2pi/35)

Answer 2

The expression is:
cos (π / 5) cos (π/7) + sin (π/5) sin(π/7)

This expression can be reduced into a trigonometric function with one angle if we make use of the trigonometric identities. The appropriate identity is:
cos (A - B) = cos A cos B + sin A sin B

If we let
A = π / 5
B = π / 7

Therefore,
cos (π / 5) cos (π/7) + sin (π/5) sin(π/7) = cos (π/5 - π/7) = cos (2π/35)