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Daca x+3y =49 si x+5y=81 determinati x +4y=?

Răspuns :

adunam relatiile
x+3y+x+5y=49+81
x+x+3y+5y=130
2x+8y=130
impartim relatia la 2 si vom avea
2x+8y = 130..\:2
x+4y=130:2
x+4y=65