sin⁴x+cos⁴x=cos²2x+¼
(sin²x-cos²x)²+2sin²x•cos²x=cos²2x+¼
cos²2x+2sin²x•cos²x=cos²2x+¼
½•sin²2x=¼
sin²2x=½
sin2x=±1/√2
2x=π/4+(π/2)•n
x=π/8+(π/4)•n, n∊Z.
