Question

в арифметической прогрессии a2=30 a8=60 Найдите a5

Answer 1

a1 + d = 30
a1 + 7d = 60
--------------------  (2) - (1)
7d - d = 60 - 30
6d = 30
d = 5

a1 = 30 - d = 25

a5 = a1  + 4d = 25 + 4*5 = 25 + 20 = 45