a₁=1 d=7 An=71 n=?
An=a₁+(n-1)*d=71
1+(n-1)*7=71
1+7n-7=71
7n=77 |÷7
n=11.
ответ:n=11.
71 = 1 + (n - 1) · 7
70 = (n - 1) · 7
n - 1 = 10
n = 11
a₁₁ = a₁ + 10d = 1 + 10 · 7 = 1 + 70 = 71
11
