Question

【在线等||求速度】设无穷等比数列{an}的各项和S=3,a2=-4/3,则lim[a1+a3+a5+...+a(2n-

Answer 1

设公比是r,则|r|＜1.

S=a1/(1-r)=3,a2=a1*r=-4/3,相除得r=-1/3,所以a1=4.

lim[a1+a3+a5+...+a(2n-1)]=a1/(1-r^2)=9/2