2sin (x - 4 / pi) sin (x + 4 / pi)

2sin (x - 4 / pi) sin (x + 4 / pi)


= 2sin (x - pi / 4) cos [pi / 2 - (x + pi / 4)]
= 2sin (x - pi / 4) cos (pi / 4 - x)
= 2sin (x - pi / 4) cos (x - pi / 4)
= 2sin [2 (x - pi / 4)]
= 2sin (2x - pi / 2)
= - 2cos2x