Арифметическая прогрессия

А3=9

А7=1

d=?

Sn=?

A5 = (A3+A7) / 2 = (9+1) / 2=5

A4 = (A3+A5) / 2 = (9+5) / 2=7

d=A4-A3=7-9=-2

An=A1+d (n-1)

A3=A1+2d

A1=A3-2d=9-2 * (-2) = 9+4=13

An=A1+d (n-1) = 13-2 (n-1) = 13-2n+2=15-2n

Sn = (A1+An) n/2 = (13+15-2n) n/2 = (28-2n) n/2 = (14-n) n=14n-n²