åé¢ï¼è®¾M为é¨åæ£æ´æ°ç»æçéåï¼æ°åï½anï½çé¦é¡¹a1 = 1ï¼ån项å为Snï¼å·²ç¥å¯¹ä»»ææ´æ°kå±äºMï¼å½nï¼kæ¶ï¼S(n+k)+S(n-k)=2(Sn+Sk)é½æç«ã
设M =ï½3ï¼4ï½ï¼æ±æ°åï½anï½çé项å
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ç½ä¸èéççæ¡ï¼å½kâ M =ï½3,4ï½ä¸nï¼kæ¶ï¼Sn+k + Sn -k = 2Sn + 2Skä¸Sn+1+k + Sn +1-k = 2Sn+1 + 2Sk,ï¼ä¸¤å¼ç¸åå¾an+1+k + an +1 -k = 2an+1ï¼å³an+1+k - an+1 = an+1 - an +1 -k .æ以å½nâ¥8æ¶ï¼an - 6, an - 3, an, a n+ 3, an+ 6æçå·®æ°åï¼ä¸an - 6, an - 2, an + 2, an + 6ä¹æçå·®æ°å.
ããã为ä½è¦ä»¥8为ç线å¢ï¼ä¸»è¦æ¯æ³ä½¿å¾nåå«å3å4æ¶æççå·®æ°åæå
±åçç差项æ°ï¼ä¸ç¶ä¸ç´æ¥ä»¤K=3ï¼æè
K=4å¢ï¼å¹²åè¦è¿æ ·ç¦å¢ï¼æ£å¥½ï¼å½nâ¥8æ¶ï¼æäºå
±åç项æ°aï¼n+6ï¼ããã
å
æa(n+1+k) - a(n+1) = a(n+1) - a(n +1 -k)转å为a(n+1+k) +a(n +1 -k)=2a(n+1).
å 为kâ M =ï½3,4ï½ï¼æ以å½k=3æ¶ï¼å³å½nï¼k=3æ¶ï¼a(n+4)+a(n-2)=2a(n+1)
å½nï¼4æ¶ï¼a(n+3)+a(n-3)=2anï¼å½nï¼5æ¶ï¼a(n+2)+a(n-4)=2a(n-1ï¼,å½nï¼6æ¶ï¼a(n+1)+a(n-5)=2a(n-2ï¼,ï¼å½nï¼7æ¶ï¼an+a(n-6)=2a(n-3),å½nï¼7æ¶ï¼åan,a(n-3),a(n-6)æçå·®æ°åãæ¨åºï¼å³nâ¥8æ¶ï¼a(n+6),a(n+3),an,a(n-3),a(n-6)æçå·®æ°å.
æ以åå½k=4æ¶ï¼å³å½nï¼k=4æ¶ï¼a(n+5)+a(n-3)=2a(n+1)ï¼å½nï¼5æ¶ï¼a(n+4)+a(n-4)=2anï¼
å½nï¼6æ¶ï¼a(n+3)+a(n-5)=2a(n-1)ï¼å½nï¼7æ¶a(n+2)+a(n-6)=2a(n-2)ï¼å½nï¼7æ¶ï¼åa(n+2)ï¼a(n-2)ï¼a(n-6)æçå·®æ°å.åæ¨åºï¼å³nâ¥8æ¶ï¼a(n+6),a(n+2),a(n-2),a(n-6)æçå·®æ°å.
â¦â¦åé¢nâ¥8æ¶ï¼a(n+2)-an=an-a(n-2),å½nâ¥9æ¶ï¼a(n+1)-a(n-1)=a(n-1)-a(n-3)ï¼å³a(n+1)+a(n-3)=2a(n-1),å³nâ¥9æ¶ï¼a(n+3),a(n+1),a(n-1),a(n-3)æçå·®æ°å.
ãè¿ä¸ªæ¹æ³ä¸å¥½ï¼æç¹åå¨æ¼åï¼ç½ä¸è¿æå¦å¤ä¸ç§è§£æ³ï¼å¦ä¸ï¼ã
Sn + 3 + Sn -3 = 2(Sn+ S3), Sn + 4+ Sn -2 = 2(Sn + 1+ S3)an + 4 + an -2 = 2an + 1(nâ¥4)
æ°åï½a3n -1ï½ãï½a3nï½ãï½a3n + 1ï½(nâ¥1)é½æ¯çå·®æ°å
Sn- a1为ä¸ä¸ªçå·®æ°ååè¥å¹²é¡¹ä¹åçåSn = an2 + bn + cï¼aãbãc为常æ°ï¼ï¼
S1 = a1, Sn + 3 + Sn - 3 =2(Sn+ S3), Sn + 4 + Sn - 4=2(Sn+ S4) a + b + c = 1, 3b + c = 0, 4b + c = 0ï¼a = 1, b = c = 0Sn = n2 an = Sn - Sn - 1ï¼S0 = 0ï¼= n2 -ï¼n -1ï¼2 = 2n -1.
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