设数列{an}的通项公式an=-n^2+10n+11,前n项和为Sn,则当Sn最大时,n=
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![设数列{an}的通项公式an=-n^2+10n+11,前n项和为Sn,则当Sn最大时,n=](/uploads/image/z/14597622-54-2.jpg?t=%E8%AE%BE%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E9%80%9A%E9%A1%B9%E5%85%AC%E5%BC%8Fan%3D-n%5E2%2B10n%2B11%2C%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2C%E5%88%99%E5%BD%93Sn%E6%9C%80%E5%A4%A7%E6%97%B6%2Cn%3D)
设数列{an}的通项公式an=-n^2+10n+11,前n项和为Sn,则当Sn最大时,n=
设数列{an}的通项公式an=-n^2+10n+11,前n项和为Sn,则当Sn最大时,n=
设数列{an}的通项公式an=-n^2+10n+11,前n项和为Sn,则当Sn最大时,n=
an=-n^2+10n+11
a1=20>0
an=-n^2+10n+11
=-(n-5)²+36
当(n-5)²<36时,
an=-(n-5)²+36>0
当(n-5)²>36时,
an=-(n-5)²+36<0
当n=11时,an=0
当Sn最大时,有:n=10,11
an=-n^2+10n+11>=0
n^2-10n-11<=0
(n-11)(n+1)<=0
-1<=n<=11
即1<=n<=11时,an>=0
其中a11=0
a10>0
所以S10=S11
S12=S11+a12
a12<0,所以S12
所以S9
an=-n^2+10n+11
a1=20>0
an=-n^2+10n+11
=-(n-5)²+36
当(n-5)²<36时,
an=-(n-5)²+36>0
当(n-5)²>36时,
an=-(n-5)²+36<0
当n=11时,an=0
当Sn最大时,有:n=10,11