Question

дано х3=3 х7=23
Знайти: S10

Answer 1

$d = \frac{23-3}{7-3} = \frac{20}{4} = 5$
$a_3 = a_1 + 2d \\ 3 = a_1 + 10 \\ a_1 = -7 \\ a_{10} = -7 + 45 = 38\\ S_{10} = \frac{-14+38}{2}*10 = 120$