30 БАЛЛОВ!1! х+2х+х...+20х=2100

Question

30 БАЛЛОВ!1! х+2х+х...+20х=2100

Answer 1

A1 = x
an = 20x
n=20
Sn = n(a1 + an) / 2 = 2100
20(x + 20x) / 2 = 2100
21x = 210
x = 10