(x-40)/39+(x-39)/40=39/(x-40)+40/(x-39)

[40(x-40)]/(39*40)+[39(x-39)]/(39*40)=

=[39(x-39)]/[(x-40)(x-39)]+[40(x-40)]/[(x-40)(x-39)]

[40(x-40)+39(x-39)]/(39*40)=

=[39(x-39)+40(x-40)]/[(x-40)(x-39)]

1/(39*40)=1/[(x-40)(x-39)]

[(x-40)(x-39)]=39*40

x^2-39x-40x+39*40=39*40

x^2-79x=0

x(x-79)=0

x=0 или x=79