Дано: d=4 an=50 Sn=330. найдите a1 и n

An=a1 + (n-1) d

50 = a1 + (n-1) * 4

a1=54-4n

Sn = (a1+an) / 2*n

330*2 = (a1+50) * n

a1=660/n - 50

660/n - 50 = 54-4n|*n

4n^2-104n+660=0|^4

n^2-26n+165=0

(n-11) (n-15) = 0

1) n=11

a1=54-44=10

2) n=15

a1=54-60=-6