Because a 1, a2 and a4 are geometric series.
So a2 2 = a1* a4.
(a 1+d)^2=a 1*(a 1+3d)
a 1^2+2a 1d+d^2=a 1^2+3a 1d
a 1d=d^2
a 1=d
Obtain a certificate
2) Because
s 10 =((n(n+ 1))/2)* d = 1 10
55d= 1 10
d=2
So an=2n.