∵a1=1，a2=2，∴ a1a2 = 2 ．又{ anan+1 }是以 1 2 为公比的等比数列，∴ anan+1 = 2 ×( 1 2 )n?1=2 3 2 ?n．∴anan+1＝23?2n，∴ an+1an+2 anan+1 = 23?2(n+1) 23?2n =2-2= an+2 a
