Mohon bantuannya ya. Terima kasih

TriGonoMetRi

a + b = π/3
cos (a + b) = cos a cos b - sin a sin b
cos (a - b) = cos a cos b + sin a sin b
---------------------------------------------------- ( + )
cos (π/3) + cos (a - b) = 2 cos a cos b
1/2 + cos (a - b) = 2 . 5/8
cos (a - b) = 5/4 - 1/2
cos (a - b) = 3/4