Question

Найдите сумму двенадцати первых членов арифметической прогрессии: 32,29,26.​

Answer 1

a1 = 32
a2 = 29
d = a2 - a1 = 29 - 32 = - 3

Sn = (a1 + an)/2* n

S12 = (a1 + a12)/2 * 12 = 6*(a1 + a12);
a12 = a1 + 11d = 32 - 33 = - 1

S12 = 6*(32 - 1) = 6*31 = 186