1)В арифметической прогрессии(an): a1=3,d=-2. Найдите a13, s13 -?2)В арифметической прогрессии: -63, -58, -53... найдите

1) a1 = 3, d = -2.

a13 = 3 + (-2) * (13 - 1) = 3 - 2 * 12 = 3 - 24 = -21;

S13 = (3 + (-21))/2 * 13 = -18/2 * 13 = -9 * 13 = -117;

2) -63, -58, -53...

d = -58 + 63 = 5;

an = -63 + 5 * (n - 1) = -63 + 5n - 5 = -68 + 5n = 5n - 68;

5n - 68 < 0, n < 13,6 => n = 13;

S13 = (2 * (-63) + 5(13 - 1))/2 * 13 = (-126 + 5 * 12)/2 = -63 + 30 = -33;

3) a1 = 30, an = 100, n = 71;

Sn = (a1 + an)/2 * n = (30 + 100)/2 * 71 = 4615.