作业帮设等差数列{an}前n项和为Sn,a3=4.则S5=
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设等差数列{an}前n项和为Sn,a3=4.则S5=
设等差数列{an}前n项和为Sn,a3=4.则S5=
设等差数列{an}前n项和为Sn,a3=4.则S5=
S5=5a1+10d=5(a1+2d)=5a3=5×4=20
因为是等差数列，a3=a1+2d，S5=(a1+a5)/2*5=（a1+a1+4d)/2*5=(a1+2d)*5=15
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