解ï¼
å ç符å·æ¯ä¸æ£ä¸è´ï¼ç¨-(-1)^k表示
åå1,3,5,7,9â¦â¦ç¬¬k个æ°ååæ¯2k-1
åæ¯2,4,8,16,32â¦â¦ç¬¬k个æ°åæ¯æ¯2^k
æ以ååºç¬¬k个æ°æ¯
-(-1)^k x (2k-1)/(2^k)= (1-2k)/(-2)^k
说æï¼^æ¯æ¬¡æ¹ç¬¦å·ï¼^kå°±æ¯k次æ¹