x^2√(a^2-x^2)dx=a^4sin^2tcos^2tdt=a^4（1-cos4t）/4dt
2.∫arcosxdx=∫tdcost=tcost-sint+c=xarcosx-√(1-x^2)+c
3.lnx=t
(lnx)^3dx =(t)^3dtt