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=[ sin^2(x)]^2
= ( (1-2cosx)/2 )^2
= 1/4 (1 - 2cos2x +cos^2(x) )
= 1/4 (1 - 2cos2x + (1+cos4x)/2 )
= 1/4 - 1/2cos2x + 1/8 + 1/8cos4x
= 3/8 - 1/2cos2x +1/8cos4x
= 1/8 (3 - 4cos2x +cos4x)
---> ∫sin^4(x)dx
= 1/8∫(3 - 4cos2x + cos 4x) dx
= 1/8 {3x - 4(sin2x)/2 +1/4sin4x} +C
=> (3/8)x - (1/4)sin2x + (1/32)sin4x + C
(C = konstanta)
Semoga membantu^^