Question

Sa se calculeze suma tuturor numerelor naturale de doua cifre care se divid cu 11. Dau coronita.

Answer 1

11 + 22+33+... + 99 = 11· ( 1 +2+3+ .. + 9 ) = 11 · ( 1 + 9)  ·9 / 2 =
= 11 · 10 · 9 / 2 =11· 5 ·9 = 495

Answer 2

   
$S = 11 + 22+33+44+55+66+77+88+99 = \\ \\ = \frac{9(99+11)}{2} =\frac{9*110}{2} =9*55=\boxed{495}$