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8a^2.∫(0->π/2) (cost)^4 dt
=2a^2.∫(0->π/2) (1+cos2t)^2 dt
=2a^2.∫(0->π/2) [1+2cos2t + (cos2t)^2 ] dt
=a^2.∫(0->π/2) ( 3+4cos2t + cos4t ) dt
=a^2. [ 3t+2sin2t + (1/4)sin4t ]|∫(0->π/2)
=a^2. (3π/2)
=2a^2.∫(0->π/2) (1+cos2t)^2 dt
=2a^2.∫(0->π/2) [1+2cos2t + (cos2t)^2 ] dt
=a^2.∫(0->π/2) ( 3+4cos2t + cos4t ) dt
=a^2. [ 3t+2sin2t + (1/4)sin4t ]|∫(0->π/2)
=a^2. (3π/2)
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