Question

(2+4+6+...+20)-(1+3+5+...+19)

Answer 1

2+4+6+...+20 = 2(1+2+3+...+10) = 2*55 = 110 Pt suma din paranteza aplic formula sumei lui Gauss => 10*11/2=55 La cea de-a doua suma observam ca merge din 2 in 2 = 1+3+5+7+9+11+13+15+17+19 = 100 => 110-100 = 10