Question

What is the value of `cos(pi/7) cos(pi/5)-sin(pi/7) sin(pi/5)`?

Answer 1

`cos ((12pi)/35)`

Explanation:

Apply the trig identity:
cos (a =b) = cos a.cos b - sin a.sin b.
`cos (pi/7)cos (pi/5) - sin (pi/7).sin (pi/5) = cos (pi/7 + pi/5) =`
`= cos ((12pi)/35) = cos 61^@71 = 0.47`