Question

若等差数列{an}的公差d=-2,且a1+a4+a7+……+a31=50,则a2+a6+a10+……+a42为?

Answer 1

a1+a4+a7+...+a31

=(a1+a31)+(a4+a28)+(a7+a25)+(a10+a22)+(a13+a19)+a16

=11a16

=50

所以：a16=50/11；

a2+a6+...+a42

=(a2+a42)+(a6+a38)+(a10+a34)+(a14+a30)+(a18+a26)+a22

=11a22

=11(a16+6d)

=11(50/11-12)

=50-132

=-82