X的平方-1分之2=X+1分之A-X-1分之B
2/(x^2-1)=A/(x+1)-B/(x-1)
2/(x^2-1)=[x+1-(x-1)]/(x^2-1)
=[(x+1)-(x-1)]/[(x+1)(x-1)]
=1/(x-1)-1/(x+1)
因等式左右相等,根据待定系数法可知
A/(x+1)=-1/(x+1) A=-1
-B/(x-1)=1/(x-1) B=-1