:1/(x+4)+1/(x+7)=1/(x+5)+1/(x+6)
1/(X+4)-1/(X+5)=1/(X+6)-1/(X+7)
(X+5-X-4)/(X²+9X+20)=(X+7-X-6)/(X²+13X+42)
X²+9X+20=X²+13X+42
X=-11/2
:1/(x+4)+1/(x+7)=1/(x+5)+1/(x+6)
1/(X+4)-1/(X+5)=1/(X+6)-1/(X+7)
(X+5-X-4)/(X²+9X+20)=(X+7-X-6)/(X²+13X+42)
X²+9X+20=X²+13X+42
X=-11/2