Notice that
• π/2 = π/3 + π/6
• π/6 = π/3 - π/6
Recall the angle sum identities for sine:
sin(x + y) = sin(x) cos(y) + cos(x) sin(y)
sin(x - y) = sin(x) cos(y) - cos(x) sin(y)
By adding these together, we get
sin(x + y) + sin(x - y) = 2 sin(x) cos(y)
==> sin(x) cos(y) = 1/2 (sin(x + y) + sin(x - y))
Now take x = π/3 and y = π/6 :
sin(π/3) cos(π/6) = 1/2 (sin(π/2) + sin(π/6))
So the blank should be filled with cos.
