Арифметическая прогрессия.
А2=1;
а4=9.
Найти:
d-?;
a1-?;
S20-?

d = (a_4 - a_2) / (4 - 2) <=> d = (9 - 1) / 2 <=> d = 8 / 2 <=> d = 4

a_2 = a_1 + d <=> a_1 = a_2 - d <=> a_1 = 1 - 4 <=> a_1 = -3

a_20 = a_1 + (20 - 1) * d <=> a_20 = a_1 + 19 * d <=> a_20 = -3 + 19 * 4 <=> a_20 = -3 + 76 <=> a_20 = 73

S_20 = (a_1 + a_20) * (20 - 1 + 1) / 2 <=> S_20 = (-3 + 73) * 20 / 2 <=> S_20 = 70 * 20 / 2 <=> S_20 = 1400 / 2 = 700

ответ: 4 ; -3 ; 700