ln(x^2 -1)=ln(x+1)+ln(x-1)
∫ ln(x^2 -1)dx =∫ln(x+1)d(x+1)+∫ln(x-1)d(x-1)
分部积分：
原式=(x+1)ln(x+1)-∫(x+1)d(ln(x+1))+(x-1)ln(x-1)-∫(x-1)d(ln(x-1))=(x+1)ln(x+1)-∫(x+1)*1/(x+1)dx+(x-1)ln(x-1)-∫(x-1)*1/(x-1)dx=(x+1)ln(x+1)+(x-1)ln(x-1)