Respuesta :

Matheng
The rule to solve this problem:
sin x sin y = 0.5 [ cos (x-y) - cos (x+y) ]

∴ If x = π/4  and   y = π/6
    x-y = π/12     and    x+y = 5π/12

∴ sin (π/4) sin (π/6) = 0.5 [ cos (π/12) - cos (5π/12) ]