x²-9=(x+3)(x-3)x²+5x+6=(x+2)(x+3)最简公分母是(x+2)(x+3)(x-3)所以2x/(x²-9)=2x(x+2)/(x+2)(x+3)(x-3)(x+2)/(x²+5x+6)=(x+2)(x-3)/(x+2)(x+3)(x-3)