已知Sn是等比数列{an}的前n项和,S3,S9,S6成等差数列,且a2+a5=2a,则m=
问题描述:
已知Sn是等比数列{an}的前n项和,S3,S9,S6成等差数列,且a2+a5=2a,则m=
a2+a5=2am打漏了·~
答
S3=a1(1-q^3)/(1-q),S9=a1(1-q^9)/(1-q).S6=a1(1-q^6)/(1-q)
呈等差数列,则q^3+q^6=2*q^9.同除以q^2,则q+q^4=2*q^7,即可求