Question

设等差数列an的前n项和为Sn,若Sm=0,Sm+2=2,Sm+4=8,则公差d=

Answer 1

S(m+2)=Sm+a(m+1)+a(m+2)

=Sm+a1+m*d+a1+(m+1)*d

=Sm+2a1+(2m+1)*d

S(m+4)=S(m+2)+2a1+(2m+5)d

S(m+4)- S(m+2)=S(m+2)-Sm+4d=6

即:2+4d=6

所以公差d=1

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