a)a=2.5 d= -2
a6= 2.5+0.5(6-1)
a6=5
б)a10= -24.6. d= -2
-24.6=a1-2(10-1)
a-18= -24.6
a1= -6.6
в)a10=20.4. a-18=22
20.4=a1+d(10-1)
22=a+d(18-1)
{a+9d=20.4
{a+17d=22
{8d=1.6
{a+1.8=20.4
{d=0.2
{a1=18.6
