来自贾维嘉的问题
等差数列{an}中,a1=1,a3+a5=14,其前n项和Sn=100,则n=______.
等差数列{an}中,a1=1,a3+a5=14,其前n项和Sn=100,则n=______.
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等差数列{an}中,a1=1,a3+a5=14,其前n项和Sn=100,则n=______.
等差数列{an}中,a1=1,a3+a5=14,其前n项和Sn=100,则n=______.
a3+a5=2a1+6d=14,
∴d=2
Sn=na1+n(n−1)d2