(x+1)/(x+2)-1/(x+7)=（x+2)/（x+3)-1/(X+6）
(X+2-1)/(X+2)-1/(X+7)=(X+3-1)/(X+3)-1/(X+6)
1-1/(X+2)-1/(X+7)=1-1/(X+3)-1/(X+6)
1/(X+2)+1/(X+7)=1/(X+3)+1/(X+6)
1/(X+2)-1/(X+3)=1/(X+6))-1/(X+7)
1/(X+2)(X+3)=1/(X+6))(X+7)
(X+2)(X+3)=(X+6))(X+7)
X^2+5X+6=X^2+13X+42
解得X=-9/2