1/（x-2)（x-3)=[1/(x-2)]-[1/(x-3)]
1/(x-1)(x-2)=[1/(x-1)]-[1/(x-2)]
于是1/（x-2)（x-3)+1/(x-1)(x-2)=[1/(x-1)]-[1/(x-3)]=1/(x-1)(x-3)
所以原式=-1/(x-1)(x-3)