Question

Арифметическая прогрессия 14,17,20..….56 найти n

Answer 1

А1 = 14
d=3
an = 56
an = a1 + d(n-1)
56 = 14 +3(n-1)
56 = 14 + 3n -3
3n = 45
n = 15

Answer 2

A1=14,a2=17,an=56
d=a2-a1=17-14=3
an=a1+(n-1)d
14+3(n-1)=56
3(n-1)=42
n-1=14
n=15