左边=6x/(x^2-9)=6x/[(x+3)(x-3)]
右边=A/(x+3)+B/(x-3)=[A(x-3)+B(x+3)]/[(x+3)(x-3)]
=[(A+B)x-3(A-B)]/[(x+3)(x-3)]
故A+B=6, A-B=0
解A=B=31除以a加上a除以x=1除以b加上b除以a