Question

в арифметической прогрессии а5=3 и а9=15 наидите сумму первых 30 членов

Answer 1

a5 = a1 + 4d = 3
a9 = a1 + 8d = 15

a1 + 4d = 3
a1 + 8d = 15
-------------------- (2) - (1)
8d - 4d = 15 - 3
4d=  12
d = 3

a1 = 3 - 4d = 3  - 12 = - 9
a30 = a1 + 29d = - 9 + 29*3 = 78

S30 = 15*(a1 + a30) = 15*69 = 1035