Question

дано арифметическая прогрессия a1=5;a2=11.Найти: a7;S7

Answer 1

a(1) = 5

a(2) = 11

a(7) -?

S(7) -?

d = a(2) - a(1)

d = 11 - 5 = 6

a(n) = a(1) +d(n-1)

a(7) = a(1) +d * 6

a(7) = 5 + 6*6 = 5+36 = 41

S(n) = [a(1) + a(n)] / 2 * n

S(7) = [a(1) + a(7)] / 2 * 7

S(7) = (5 + 41) / 2 * 7 = 46 / 2 * 7 = 23 * 7 = 161