馍夹肉
春芽
共回答了16个问题采纳率:68.8% 举报
解题思路:(1)通过读表得到表达式,利用等差等比数列的通项公式将表达式展开,求出
![](https://img.yulucn.com/upload/0/6a/06adbed0e1485fe1f52826c9a3084512_thumb.jpg)
,得到数列
![](https://img.yulucn.com/upload/6/bf/6bf936cd480afa952223b0e8b8d749e4_thumb.jpg)
的通项公式;(2)将第一问的结论代入,先用分组求和法,将式子分成两组,再用错位相减法求第一部分,第二部分用并项法求和.
试题解析:(1)设第一行依次组成的等差数列的公差是
![](https://img.yulucn.com/upload/c/eb/ceba725e51da2d57f5b4fbc414bdfb48_thumb.jpg)
,等比数列的公比是
![](https://img.yulucn.com/upload/d/52/d5254ad6099a57643d675c91f69f9617_thumb.jpg)
,
则
![](https://img.yulucn.com/upload/8/44/844fe217d0e802006c32eadcbb23005a_thumb.jpg)
, 2分
![](https://img.yulucn.com/upload/0/9e/09ec0ce2cf862bb21675fc94f2acd5d8_thumb.jpg)
, 4分
解得:
![](https://img.yulucn.com/upload/0/6a/06adbed0e1485fe1f52826c9a3084512_thumb.jpg)
,所以:
![](https://img.yulucn.com/upload/6/16/616248cc24f530a111fe7cd5431dbfde_thumb.jpg)
; 6分
(2)
![](https://img.yulucn.com/upload/8/ef/8ef786f575e30bc6f229ac8090937bde_thumb.jpg)
,
![](https://img.yulucn.com/upload/4/bd/4bd98b48cfed25f2beae105e51302fbe_thumb.jpg)
, 8分
记
![](https://img.yulucn.com/upload/f/c0/fc009a964a686a0ab2c514ff51b9daf8_thumb.jpg)
,则
![](https://img.yulucn.com/upload/9/44/944fc3b5ed4aa0cd5dc0f4427198f2c9_thumb.jpg)
,
两式相减得:
![](https://img.yulucn.com/upload/8/47/8472fc8d0f796409e09e04ff1534017a_thumb.jpg)
,所以
![](https://img.yulucn.com/upload/0/e1/0e10a09495521b328bedca968111f20d_thumb.jpg)
, 10分
所以
![](https://img.yulucn.com/upload/b/77/b77962b6528c486fe47f1135e8e3b51e_thumb.jpg)
为偶数时,
![](https://img.yulucn.com/upload/a/91/a91185c30c7ea24fbed6329e6f4c2993_thumb.jpg)
,
![](https://img.yulucn.com/upload/b/77/b77962b6528c486fe47f1135e8e3b51e_thumb.jpg)
为奇数时,
![](https://img.yulucn.com/upload/8/d7/8d756107871af4462ccdb04c0ea12a83_thumb.jpg)
。 12分
(1)
;(2)
为偶数时,
,
为奇数时,
.
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1年前
2