
We want f'(x)
So, lets derive, we can consider sin(x) as an normal x and derive it as x² going to 2x, and apply chain rule to sin(x)

So just use x as pi/2
f'(π/2) = π/2 . sin^(π/2-1)(π/2) . cos^(π/2).(π/2)
sin(π/2) = 1
cos(π/2) = 0
f'(π/2) = π/2 . 1^(π/2 - 1) . 0^(π/2)
f'(π/2) = 0