A3=20 A8=100 Найти d a1

Question

A3=20
A8=100
Найти d a1

Answer 1

A8=a3+5d
100=20+5d
100-20=5d
80=5d
d=80:5
d=16

a3=a1+2d
20=a1+2×16
20=a1+32
a1=32-20
a1=12