Determina numarul natural x pentru care 7×x+14×x+21×x+28×x+......+126×x=7×9×19

Răspuns :

7×x+14×x+21×x+28×x+......+126×x=7×9×19
7x ( 1 + 2 + 3 + 4 +...+ 18) = 7 × 9 × 19
7x × 18 × 19 : 2 = 7 × 9 × 19
7x × 9 × 19 = 7 × 9 × 19
x = 1