令上下都乘4再令u=2x^2
∫4xdx/4√(1-2x^2)=(1/4)d(2x^2)=0.25∫d(2x^2)/√(1-x^2)=0.25∫d(2x^2)/√(1-x^2)=0.25∫du/√(1-U)乘下的自己做吧.