RQW@ @pþÝn¾aì@pbÉibnn¾a
r =1 an = a1rn−1 ()
3
9, 3,1, 1 , 1,…
39
.a1 = 9
1 n −1
3
an = a1r n−1 = 9 × = 32 × 3−n+1 = 33−n
.n ≥ 1 an = 33−n
k, 3k,20 − k ()
k
3k = 20 − k .k = 2
k 3k ()
3 = 20 − k
3k
9k = 20 − k
10k = 20
6, 6 2, 12, 12 2, …
. 1400
( ).an = 6 × n−1 .r = 2 a1 = 6
2