已知Sn是等差数列{An}的前n项和,S9=18,Sn=240,A(n-4)=30(n>9),求n的值!
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已知Sn是等差数列{An}的前n项和,S9=18,Sn=240,A(n-4)=30(n>9),求n的值!
已知Sn是等差数列{An}的前n项和,S9=18,Sn=240,A(n-4)=30(n>9),求n的值!
已知Sn是等差数列{An}的前n项和,S9=18,Sn=240,A(n-4)=30(n>9),求n的值!
S9=18 得出 A5=2
a1=a5-4d 得出 a1=2-4d
a(n-4)=30 得出 an=a(n-4)+4d =30+4d
sn=(a1+an)n/2 =(2-4d+30+4d)*n/2=16*n=240
n=15
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