令 x = 2sint则 I = ∫<0, π/3> 4(sint)^2 (2costdt)/(2cost)= ∫<0, π/3> 4(sint)^2dt = 2∫<0, π/3> (1-cos2t)dt= 2[t+(1/2)sin2t]<0, π/3> = 2[π/3+√3/4] = 2π/3+√3/2
