Posted by kc on Sunday, July 24, 2011 at 8:47pm.
sin4x=-2sin2x
2 sin2xcos2x = -2sin2x, ......... since sin2A = 2sinAcosA
cos2x = -1
2x = 180°, 360°, 540°, 720° which is π , 2π 3π, or 4π radians
x = 90° ,180°,270°, 360° which is π/2, π , 3π/2, or 2π radians