1. a3=a1+(3-1)d=a1+2d=-5,2+2*6=-5,2+12=6,8
a8=a1+7d=-5,2+42=36,8
a(n+2)=a1+(n+2-1)d=a1+(n+1)d=-5,2+(n+1)d
2. a1=6
a21=a1+20d=42 => 6+20d=42 => 20d=42-6 =>20d=36 => d = 36/20 = 18/10
d = 1,8
3. a1=14, a2 = 17
a2=a1+d => d = a2-a1 => d = 17-14=3
an=50
a1+(n-1)d=50
14+3(n-1)=50
14+3n-3=50
11+3n=50
3n=50-11
3n=39
n=39/3=13
Ответ: n = 13