①2mn/4m^2-9=2mn/[(2m+3)(2m-3)]
②2m-3/2m+3=(2m-3)(2m-3)/[(2m+3)(2m-3)]=(2m-3)²/[(2m+3)(2m-3)]
③2mn/4m²-9与2m-3/2m+3=2mn-(2m-3)²/[(2m+3)(2m-3)]
最简公分母是[(2m+3)(2m-3)]