1) S(15,30) = S30 - S14
S14 = (2a1 + d*(n - 1))/2 * n = (2*12 + 13*7)/2 * 14 = 805.
S30 = (2a1 + d*(n - 1))/2 * n = (2*12 + 29*7)/2 * 30 = 3405
S(15,30) = 3405 - 805 = 2600.
Ответ: 2600
2) a1 = 15,
d = 22 - 15 = 7
S8 = (2a1 + d*(n - 1))/2 * n = (2*15 + 7*7)/2 * 8 = 316.
Ответ: 316.