等差数列{an}的前n项和为Sn,若S12=84,S20=460,求S10

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等差数列{an}的前n项和为Sn,若S12=84,S20=460,求S10
等差数列{an}的前n项和为Sn,若S12=84,S20=460,求S10

等差数列{an}的前n项和为Sn,若S12=84,S20=460,求S10
设an=a1+(n-1)d
sn=n[2a1+(n-1)d]/2
s12=12(a1+11d/2)=84,a1+11d/2=7
s20=20(a1+19d/2)=460,a1+19d/2=23
d=(23-7)/4=4
a1=7-44/2=-15
a10=-15+4x(10-1)=21
s10=(-15+21)x10÷2=30

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