x*y=xy-5x-5y+30. Trbuie sa arat ca x*y=(x-5)(y-5)+5

Răspuns :

x*y=xy-5x-5y+30=
(xy-5x)+(-5y+5)+25
x(y-5)-5(y-5)+25=
(y-5)(x-5)+25 QED