1/(x²-4x) = 1/[x(x-4)] = (1/4)[1/(x-4) - 1/x]剩下的就不用多说了吧
∫dx/(x^2-4x)=∫dx/[x(x-4)]=(1/4)∫[1/(x-4) -1/x] dx=(1/4)( ln|x-4| -ln|x| ) + C
