x-15/x^2-9---3/3+x=?

(x-15)/(x^2-9)-3/(3+x)
=(x-15)/[(x+3)(x-3)]-3/(x+3)
=(x-15)/[(x+3)(x-3)]-3(x-3)/[(x+3)(x-3)]
=[(x-15)-3(x-3)]/[(x+3)(x-3)]
=(-2x-6)/[(x+3)(x-3)]
=-2(x+3)/[(x+3)(x-3)]
=-2/(x-3).

本题是一个分式的化简题，先看各自的分式能否约分，
如果不能，那么就要进行通分，然后在化简。解题如下：
(x-15)/(x^2-9)-3/(3+x)
=(x-15)/[(x+3)(x-3)]-3/(x+3)
=(x-15)/[(x+3)(x-3)]-3(x-3)/[(x+3)(x-3)]
=[(x-15)-3(x-3)]/[(x+3)(x-3)]
=(-2x-6)/[(x+3)(x-3)]
=-2(x+3)/[(x+3)(x-3)]
=-2/(x-3)
=2/（3-x）