题目

设函数y=的最小值为an，最大值为bn.若cn=(1-an)(1-bn)，则数列{cn}A.是公差不等于零的等差数列                       B.是公比不等于1的等比数列C.是常数列                                       D.不是等差数列也不是等比数列 答案：答案：C  由y=,得(y-1)x2+(y+1)x+(y-n)=0，由Δ=(y+1)2-4(y-1)(y-n)≥0，得3y2-(4n+6)y+(4n-1)≤0,由已知an、bn是关于y的方程3y2-(4n+6)y+(4n-1)=0的两根，∴an+bn=,anbn=,cn=1-(an+bn)+anbn=1.Beyond the river_____.A． lives an old fishermanB． does an old fisherman liveC．an old fisherman livesD．do an old fisherman live