设等差数列{an}的前n项和为Sn,如果a3+a19=6,那么S21=?
1个回答
a1+a21=a1+(a1+20d)=2a1+20d
a3+a19=(a1+2d)+(a1+18d)=2a1+20d
所以a1+a21=a3+a19=6
S21=(a1+a21)*21/2=63
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