В арифметической прогрессии a7=8 и a11=12,8. Найдите a1 и d.

Question

В арифметической прогрессии a7=8 и a11=12,8. Найдите a1 и d.

Answer 1

1)а11=а7+4d 2)a7=a1+6d  8=a1+6*1.2 a1=0.8
12.8=8+4d
4d=12.8-8
d=1.2

Answer 2

A1=0.8 d=1.2 -------- {a11=a1+10×d {a7=a1+6×d {12.8=a1+10×d {8=a1+6×d a1=8-6×d 12.8=8-6×d+10×d 4d=4.8 d=1.2 a1=8-6×1.2 a1=0.8