Question

Sa se calculeze suma S = 1+4+7+ ... +61

Answer 1

S = 1+4+7+ ... +61

S = 1 + (1+3) + (1+6) + ... + (1+60)

[(61-4):3 + 1 ]= [19 + 1] + 1 = 21 (cifre de 1)

S = 21×1 + 3 + 6 + ... + 60

S = 21 + 3(1 + 2 + ... + 20)

S = 21 + 3×20×21/2

S = 21 + 630

S = 651