x+y=5 / ()^2
x^2+2xy+y^2=25
x^2-y^2=45=> x^2=45+y^2
Inlocuim in prima ecuatie:
45+y^2+2xy+y^2=25
2y^2+2xy=25-45
2y^2+2xy=-20
2y(y+x)=-20 dar in problema ne zice ca x+y=5, deci inlocuim
2y*5=-20
10y=-20
y=-2
x+(-2)=5
x-2=5
x=5+2
x=7
5y-5x=
5(y-x)=
5*(-2-7)=
5*(-9)=-45 sau se putea face fara a scoate factor comun
5*(-2)-5*7=
-10-35=-45
