设x=sin(a) a取值范围为[0,pi/2] （2/3）∫(1-x^2)的3/2次方dx =（2/3）∫cos^3(a)dsin(a) =（2/3）∫cos^4(a)da =（2/3）∫cos^4(a)da =(2/3)∫[(1+cos2a)/2]^2da =(2/3)∫(1+cos^2(2a)+2cos(2a))/4da =(2/3)∫1/4d...