Question

Найти сумму и первых членов арифметической прогрессии,
если:
1) a1 = 1, an = 20, n = 50;
3) a1 = -1, an = -40, n = 20;
2) a1 = 1, an = 200, n = 100;
4) a1 = 2, an = 100, n = 50.

Answer 1

$S = \frac{a1+an}{2}*n$

1) $S = \frac{1+20}{2}*50 = 525$

3) $S = \frac{-1-40}{2}*20 = -410$

2) $S = \frac{1+200}{2}*100 = 10050$

4) $S = \frac{2+100}{2}*50 = 2550$