x平方+3x+2分之1=1/[(x+1)(x+2)]=(x+3)/[(x+1)(x+2)(x+3)] x平方+4x+3分之1=1/[(x+1)(x+3)]=(x+2)/[(x+1)(x+2)(x+3)] x平方+5x+6分之1=1/[(x+3)(x+2)]=(x+1)/[(x+1)(x+2)(x+3)]