(1-3x)/(x^2-1)=m/(x+1)+n/(x-1)
两边同时乘以(x+1)*(x-1),得：
1-3x=m(x-1)+n(x+1)=(m+n)x+(n-m)
对比系数,得：
m+n=-3
n-m=1
m=-2,n=-1