写错了,应该是
7/(2x²+6x)-（11-2x²）/(9-x²）+2
=7/[2x(x+3)]-(2x²-11)/(x²-9）+2
=7/[2x(x+3)]-(2x²-11)/(x+3)(x-3)+2
=[7(x-3)-2x(2x²-11)+2*2x(x+3)(x-3)]/[2x(x+3)(x-3)]
=(7x-21-4x³+22x+4x³-36x)/[2x(x+3)(x-3)]
=(-7x-21)/[2x(x+3)(x-3)]
=-7(x+3)/[2x(x+3)(x-3)]
=7/(6x-2x²）