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.