(x-4）/ (x-5) - (x-5)/(x-6) = (x-7) / (x-8) - (x-8) / (x-9)
对方程两边各自通分：
[(x-4)(x-6) - (x-5)(x-5)] / [(x-5)/(x-6)] = [(x-7)(x-9) - (x-8)(x-8)] / [(x-8)(x-9)]
(x^2-10x+24-x^2+10x-25) / [(x-5)/(x-6)] = [x^2-16x+63 - x^2+16x-64] / [(x-8)(x-9)]
1 / [(x-5)/(x-6)] = 1 / [(x-8)(x-9)]
所以：
(x-5)(x-6)=(x-8)(x-9)
x^2-11x+30=x^2-17x+72
6x=42
x=7
带入原方程：
左边=-1/2
右边=-1/2
左边=右边
∴x=7是原方程的解