Question

等比数列{an}中,q＝2分之一,a1＋a3＋a5＋…＋a49＝30,则a1＋a2＋a3＋…＋a50＝

Answer 1

a1(1-0.5^49)2=30+a2

a1(2-0.5^48)=30+0.5a1

a1=30/(1.5-0.5^48)

a2=15/(1.5-0.5^48)

a50=30*0.5^49/(1.5-0.5^48)

a2+a50

=15(1+0.5^48)/(1.5-0.5^48)

约等于10

a1+..a50约等于 40