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?
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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