(X-4)/(X-5)-(X-5)/(X-6)=(X-7)/(X-8)-(X-8)/(X-9)
[(x-4)(x-6)-(x-5)2]/(x-5)(x-6)=[(x-7)(x-9)-(x-8)2]/(x-8)(x-9)
(x2-10x+24-x2+10x-25)/(x2-11x+30)=(x2-16x+63-x2+16x-64)/(x2-17x+72)
整理得：
-1/(x2-11x+30)=-1/(x2-17x+72)
即：
x2-11x+30=x2-17x+72
17x-11x=72-30
6x=42
x=7