Question

Calculeaza:a) 1+2+3+.......+70.b) 11+12+13+....+99; c) 2+5+8...+302; d) 2+7+12....+502; e) 3+6+9+....123;

Answer 1

a) 70 (70 + 1 ) : 2 =
70 x 71 : 2 =
35 x 71 = 2485

e) 3+6+9+...123=
3 (1 + 2 + 3+......41) =
3 x 41 x (41+1) :2=
3 x 41 x 42 :2 =
3 x 41 x 21 =2583

 b) (99+11) x (99-11+1) ;2 =
110 x 89 :2 =
55 x 89 = 4895
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