Question

Calzuleaza 1+2+3+.......39 1+2+3+........-+40. 1+2+3+.......+50 30+31+32+.........100 12+13+14+......+110

Answer 1

x -> inmultit; / -> impartit

a)1+ 2 + 3+ ... + 39 = (39x40)/2 = 39x20 = 780

b) 1+ 2 + 3 +... + 40 = (40x41)/2 = 20x 41 = 820

c) 30 + 31 + 32 + ... + 100 = (100x101)/2 - (29x30)/2 = 50x101 - 29x15 = 5050 - 435 = 4615

d) 12+ 13 + 14 + ... + 110 = (110x111)/2 - (13x14)/2 = 55x111 - 13x7 = 6105 - 91 = 6014