因为等差数列{an}和等比数列{bn}各项都是正数，且a1=b1，a2n+1=b2n+1，所以an+1-bn+1= a1+a2n+1 2 - b1?b2n+1 = a1+a2n+1?2 a1?a2n+1 2 = ( a1 ? a2n+1 ) 2 2 ≥0．即 an+1≥bn+1．故选 B．
