微积分3到-3 2倍根号下(1-(x^2/9))dx

设x/3=sint
dx/dt=3cost
原式
=6∫(1-(sint)^2)^(1/2)×cost dt
=6∫(cost)^2 dt
=6∫1/2+(cos2t)/2 dt
=3∫cos2t+1 dt
=3[(sin2t)/2+t]
代入x=3,-3
t=π/2,-π/2
得3π