a1= 10, n = 11, Sn = 330. Найти an, d

Question

a1= 10, n = 11, Sn = 330. Найти an, d

Answer 1

Sn=(a1+an)n/2

отсюда находим an

2Sn=(a1+an)n

a1= 10, n = 11,Sn = 330

330*2=11(10+an)

660=11(10+an)

10+an=60

an=50

an=a1+d(n-1)

a1= 10, n = 11

50=10+d(11-1)

40=d10

d=4