（x+2分之x+3）-（x+1分之x+2）=（x+4分之x+5）-（x+3分之x+4） 计算x+3分之2x+9-x-3分

[(x+2)+1]/(x+2)-[(x+1)+1]/(x+1)=[(x+4)+1]/(x+4)-[(x+3)+1]/(x+3)
1+1/(x+2)-1-1/(x+1)=1+1/(x+4)-1-1/(x+3)
1/(x+2)-1/(x+1)=1/(x+4)-1/(x+3)
[(x+1)-(x+2)]/(x+2)(x+1)=[(x+3)-(x+4)]/(x+4)(x+3)
-1/(x+1)(x+2)=-1/(x+3)(x+4)
(x+1)(x+2)=(x+3)(x+4)
x²+3x+2=x²+7x+12
-4x=10
x=-5/2
检验：……
x+3分之2x+9-x-3分之1
=[(2x+9)(x-3)-(x+3)]/(x+3)(x-3)
=(2x²+3x-27-x-3)/(x+3)(x-3)
=(2x²+2x-30)/(x²-9)