:(x+3)/(x+2)-(x+5)/(x+4)=(x+7)/(x+6)-(x+9)/(x+8)1+1/(x+2)-1-1/(x+4)=1+1/(x+6)-1-1/(x+8)1/(x+2)-1/(x+4)=1/(x+6)-1/(x+8)2/(x+2)(x+4)=2/(x+6)(x+8)2(x+2)(x+4)=2(x+6)(x+8)(x+6)(x+8)=(x+2)(x+4)x^2+14x+48=x^...第二步是怎么来的？？(x+3)/(x+2)-(x+5)/(x+4)=(x+7)/(x+6)-(x+9)/(x+8){(x+2)+1}/(x+2)-{(x+4)+1}/(x+4)={(x+6)+1}/(x+6)-{(x+8)+1}/(x+8)1+1/(x+2)-1-1/(x+4)=1+1/(x+6)-1-1/(x+8)