数列

castelu

数列

战巡

<P><IMG alt="" src="http://www.imathas.com/cgi-bin/mimetex.cgi?\displaystyle b{n+1}-b_n=6" border=0> </P>
<P>因此 <IMG alt="" src="http://www.imathas.com/cgi-bin/mimetex.cgi?\displaystyle b_n=6(n-1)+b_1=6(n-1)-a" border=0></P>
<P>&nbsp;因此 <IMG alt="" src="http://www.imathas.com/cgi-bin/mimetex.cgi?\displaystyle a_{n+1}-a_1=\sum_{i=1}^nb_i" border=0></P>
<P>&nbsp;<IMG alt="" src="http://www.imathas.com/cgi-bin/mimetex.cgi?\displaystyle a_{n+1}=3(n^2-n)-na+a" border=0></P>
<P>&nbsp;<IMG alt="" src="http://www.imathas.com/cgi-bin/mimetex.cgi?\displaystyle a_{n+1}=3n^2-(3+a)n+a" border=0> </P>
<P>因为n+1=6,即n=5时原式有最小值,那么只需保证5是所有整数里离对称轴最近的就可以了</P>
<P>那么就有 </P>
<P><IMG alt="" src="http://www.imathas.com/cgi-bin/mimetex.cgi?\displaystyle 4.5<\frac{3+a}{6}<5.5" border=0></P>
<P><IMG alt="" src="http://www.imathas.com/cgi-bin/mimetex.cgi?\displaystyle 24<a<30" border=0></P>

[ 本帖最后由 战巡 于 2008-3-10 21:33 编辑 ]

castelu

完全正确