45分钟阶段测试(八)(范围:6.16.4)一、选择题1(2014辽宁)设等差数列an的公差为d.若数列2为递减数列,则()Ad0Ca1d0答案C解析设bn2,则bn12,由于2是递减数列,则bnbn1,即22.y2x是单调增函数,a1ana1an1,a1ana1(and)0,a1(anand)0,即a1(d)0,a1d对一切nN都成立的最大正整数k的值解(1)当n1时,a1S16,当n2时,anSnSn1(n2n)(n1)2(n1)n5.而当n1时,n56,符合上式,ann5(nN)(2)cn(),Tnc1c2cn(1)()().Tn1Tn0,Tn单调递增,故(Tn)minT1.令,得k671,所以kmax671.