(x+3)/(x+4)+(x+8)/(x+9)=(x+4)/(x+5)+(x+7)/(x+8)
1-1/(x+4)+1-1/(x+9)=1-1/(x+5)+1-1/(x+8)
1/(x+4)+1/(x+9)=1/(x+5)+1/(x+8)
(x+9+x+4)/[(x+4)(x+9)]=(x+8+x+5)/[(x+5)(x+8)]
(2x+13)/(x^2+13x+36)=(2x+13)/(x^2+13x+40)
由于x^2+13x+36恒不等于x^2+13x+40,要等式成立,只有
2x+13=0
x=-13/2