Question

a)a1=7,a6=25 Найти а4
б)a10=1.9 u a16=6.1 Найти а1 и d

Answer 1

1.  a1=7   a6=25
      a6=7+d*5      7+5d=25   5d=18   d=18/5=3.6
a4=7+3d=4+3*3.6=14.8

2.  a10=1.9   a16=6.1   a1,d?
a1+9d=1.9
a1+15d=6.1
15d-9d=6.1-1.9
6d=5.1  d=0.85   a1=1.9-9*0.85=1.9-7.65=-5.75