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S4=(A1+A4)×4/2=21
末四项和=(A(n-3)+An)×4/2=67
An=A1+(n-1)d=A4+(n-4)d
A(n-3)=A1+(n-4)d
末四项和=(A1+A4+2(n-4)d)×2=67
A1+A4=21/2
A1+A4+2(n-4)d=21/2+2(n-4)d=67/2
(n-4)d=23/2
Sn=(A1+An)×n/2=286
=(A1+A4+(n-4)d)×n/2
=(21/2+23/2)×n/2=286
n=26
1年前
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