So sn = na1+n (n-1) d/2 =18n-3n (n-1)/2 = 54.
After sorting, it is: n 2-13n+36 = 0, (n-4)(n-9)=0, so n=4, or n=9.
(2) In geometric series, A2 * a (n-1) = a1* an =128, while a 1+an=66.
So a 1=2, an = 64 or a 1=64, an=2.
sn=a 1( 1-q^n)/( 1-q)=(a 1-anq)/( 1-q)
When a 1=2 and an=64, sn = (2-64q)/(1-q) =126, and the solution is: q=2, then an = a1* 2 (n-
When a 1=64 and a 1=2, an = (64-2q)/(1-q) =126, and the solution is: q= 1/2, then an =