: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