Question

система x^2-y^2=72

                 x+y= 9

Answer 1

x=9-y

(9-y)^2-y^2=72

81-18y=72

18y=9

y=0,5

 

x=8,5

Answer 2

x^2-y^2=72   x=9-y

(9-y)^2-y^2=72

(81-18y+y^2)-y^2=72

81-18y+y^2-y^2=72

-18y+81-72=0

-18y+9=0

18y=9

y=18/9=0.5     x=8.5