ckjiaodeping
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设数列
![](https://img.yulucn.com/upload/9/c9/9c90cbf4c9ccc7b4a076105602ff26f2_thumb.jpg)
是集合
![](https://img.yulucn.com/upload/5/cb/5cb77e5d76504657be47b20966c413c5_thumb.jpg)
中的数从小到大排列而成,即a
1 =3,a
2 =5,a
3 =6,a
4 =9,a
5 =10,…。现将各数按照上小下大、左小右大的原则排成如下三角形表:
1、.写出这个三角形的第四行和第五行的数;
2、求a
100 ;
3、设{
![](https://img.yulucn.com/upload/9/81/9811e684c4f2926e6f81a69bb9171afb_thumb.jpg)
}是集合
![](https://img.yulucn.com/upload/d/7b/d7b6199308df161800560823c8bec1dc_thumb.jpg)
中的数从小到大排列而成,已知
![](https://img.yulucn.com/upload/8/48/848cfca2738eeff1bebe4e64935f4838_thumb.jpg)
=1160,求k的值.
1、显然第i行为t=i ( i =1,2,3,…)时,故
第四行为:17,18,20,24;
第五行为:33,34,36,40,48。
2、以三角形数表中,每行的数字个数为数列各项建立等差数列
![](https://img.yulucn.com/upload/5/92/59275f8b61a3f6f8bfdb155a11b87710_thumb.jpg)
,
则C
1 =1,C
2 =2,C
3 =3,
则有
![](https://img.yulucn.com/upload/2/b4/2b434158f77e8fa6e2904e96b27e3d96_thumb.jpg)
, 令
![](https://img.yulucn.com/upload/8/7e/87e0d9bf5a2e6af47d6bfa11e54eb8eb_thumb.jpg)
,
显然有S
13 =91<100,S
14 =105>100 故在第14行倒数第6位。
T="14," s=8∴a
100 =
![](https://img.yulucn.com/upload/3/56/356c9437e1790387e78e0eabb921e798_thumb.jpg)
="16640"
3、显然,当r取2时,可得一个数,即c
1 =2
2 +2
1 +2
0 =7;
当r取3时,可得三个数,即c
2 =2
3 +2
1 +2
0 =11,c
3 =2
3 +2
2 +2
0 =13,c
4 =2
3 +2
2 +2
1 =14;
当r取n+1时,可得个
![](https://img.yulucn.com/upload/2/63/2633c0a8f0fbdb87a14e32d4c6a7beda_thumb.jpg)
数.
以r取n+1时所得数的个数为各项建立数列
![](https://img.yulucn.com/upload/c/50/c50fce6cb03b8e3c28344cf0fb2b704e_thumb.jpg)
,
故有
![](https://img.yulucn.com/upload/b/b8/bb8c0ef3f5f012fe76a1bfbb2c529d82_thumb.jpg)
,
因为当取2、3、4、…、9时,共有
![](https://img.yulucn.com/upload/8/46/846395d05f61b388183f35a35d36969f_thumb.jpg)
个,
又r取10时比t=7,s=3时小的数共有
![](https://img.yulucn.com/upload/3/52/35226faaa6f6ca35590530de8369195c_thumb.jpg)
个,
故当
![](https://img.yulucn.com/upload/8/bc/8bc1610e4d6e2afa259c6f6fccd49a83_thumb.jpg)
时k="145"
略
1年前
10