a(n)=a+d(n-1) we are told that a(10):a(30) is 1:3
(a+9d)/(a+29d)=1/3
3a+27d=a+29d
2a=2d, a=d ....
s(n)=(a+a+d(n-1))(n/2)
s(n)=(2a+dn-d)(n/2)
s(n)=(2an+dn^2-dn)/2 since a=d:
s(n)=(2an+an^2-an)/2
s(6)=42=(12a+36a-6a)/2
84=42a
a=2 and d=2 so:
a(n)=2+2(n-1)
a(n)=2n
So the first two terms are 2 and 4
