Question

Решите прогрессию
a1 = 7
an = -385
n = 50
Найти
Sn -?
d -?

Answer 1

an=a1+d(n–1)=7+50d–50=50d–43
50d–43=–385
50d=–342
d=–6,84
S50=(a1+an)/2•50=(7–385)•25=
= –9450