积分(cost)^4dt
=积分[(cos2t+1)/2]^2dt
=1/4积分[(cos2t)^2+2cos2t+1]dt
=1/4积分[1/2(cos4t+1)+2cos2t+1]dt
=积分【1/8cos4t+1/2cos2t+3/8]dt
=(1/32)sin4t+(1/4)sin2t+(3/8)t
=(3/16)π
再乘以前面的8a^2,就是（3/2)a^2π.
积分(cost)^4dt
=积分[(cos2t+1)/2]^2dt
=1/4积分[(cos2t)^2+2cos2t+1]dt
=1/4积分[1/2(cos4t+1)+2cos2t+1]dt
=积分【1/8cos4t+1/2cos2t+3/8]dt
=(1/32)sin4t+(1/4)sin2t+(3/8)t
=(3/16)π
再乘以前面的8a^2,就是（3/2)a^2π.