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)
