Sn是等差数列an的前n项和,若S6=36,Sn=324,S(n-6)=144(n>6),则n等于?
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![Sn是等差数列an的前n项和,若S6=36,Sn=324,S(n-6)=144(n>6),则n等于?](/uploads/image/z/1131995-11-5.jpg?t=Sn%E6%98%AF%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97an%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%2C%E8%8B%A5S6%EF%BC%9D36%2CSn%EF%BC%9D324%2CS%28n-6%29%3D144%28n%3E6%29%2C%E5%88%99n%E7%AD%89%E4%BA%8E%3F)
Sn是等差数列an的前n项和,若S6=36,Sn=324,S(n-6)=144(n>6),则n等于?
Sn是等差数列an的前n项和,若S6=36,Sn=324,S(n-6)=144(n>6),则n等于?
Sn是等差数列an的前n项和,若S6=36,Sn=324,S(n-6)=144(n>6),则n等于?
S6=a1*6+6(6-1)/2*d=36,则2a1+5d=12.&
最后六项的和S=an*6-6(6-1)/2*d=6an-15d
S(n-6)=Sn-S=324-(6an-15d)=144,则2an-5d=60.@
&+@:a1+an=36
Sn=(a1+an)/2*n
n=18
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