The power of geometric series q3 =a4/a 1, and q=3. b 1=a 1=6，b3=a2= 18。 So arithmetic progression 2d=b3-b 1= 12, so b=6. The sum of the first n terms in arithmetic progression is s = a1n+n (n-1) d/2 = 6n+6n (n-1)/2 = 3n (n+1).