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RRU@ @pþÝn¾aì@pbÉibnn¾a
r
rSn = a1r + a1r 2 + + a1r n−1 + a1r n
rSn − Sn = a1r n − a1
( )Sn (r − 1) = a1 rn − 1
( )Sn
= a1 rn −1
r −1
.r =/ 1
()
9, − 3, 1, − 1 , 1, …
39
.r = −1 a1 = 9
3
( ). 9 − 1 20 − 1 − 1
a1 rn −1 3 20
= r −1 = = − 27 1
S20 −1 −1 4 3
3
90 ( )
. .24
.