A1=23 d=7 s14=?a5=?помогите

Question

A1=23 d=7 s14=?a5=?
помогите

Answer 1

S₁₄ = (2a₁ + 13d)/2 · 14 = (2·23 + 13·7)/2 · 14 = (46 + 91)·7 = 137·7 = 959
a₅ = a₁ + 4d = 23 + 4·7 = 23 + 28 = 51