等价号左边=ln(1-a*x^2)-ln(1+a*x^2)由泰勒公式展开上式= - a*x^2 - o(x^2)- a*x^2 + o(x^2) =-2a*x^2+o(x^2)等价号右边也用泰勒公式展开=6x^2+o(x^2)比较可得a=-3
