作业帮设Sn是等差数列an的前n项和,a5=2,an-4=30,Sn=136求n!
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设Sn是等差数列an的前n项和,a5=2,an-4=30,Sn=136求n!
设Sn是等差数列an的前n项和,a5=2,an-4=30,Sn=136求n!
设Sn是等差数列an的前n项和,a5=2,an-4=30,Sn=136求n!
题目有问题
Sn=(a1+an)*n/2
=[2a1+(n-1)d]*n/2
=[a1+4d+a1+(n-5)d]*n/2
=[a5+a(n-4)]*n/2
136=[2+30]*n/2
136=16n
n=8.5
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