@ @¶ìÿa@óÝy‹¾a@ u@ @RQX n 1400 .an > 1400 .a17 = 1536 a16 = 768 2 a15 = 768 a17 . . .162 −6 () . ( ) a2 = a1r = −6 ( ) a5 = a1r 4 = 162 () () a1r 4 = 162 r = −3 a1 = 2 a1r −6 () r 3 = −27 .9 r = 3 −27 r = −3 .an = 2 × (−3)n−1 20 2 .