极坐标积分范围如图
I = ∫∫
= (1/4)∫<-π/2, π/2>dt [r^4]<0, acost>
= (a^4/4)∫<-π/2, π/2>(cost)^4dt = (a^4/16)∫<-π/2, π/2>(1+cos2t)^2dt
= (a^4/16)∫<-π/2, π/2>[1+2cos2t+(cos2t)^2]dt
= (a^4/16)∫<-π/2, π/2>[3/2+2cos2t+(1/2)cos4t]dt
= (a^4/16)[3t/2 + sin2t + (1/8)sin4t]<-π/2, π/2> = 3πa^4/32
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