已知数列{an}的前n项和为Sn.满足an+Sn=2n.设bn=(2-n)(an-2),bn属于(-无穷,m),求m取值范围
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已知数列{an}的前n项和为Sn.满足an+Sn=2n.设bn=(2-n)(an-2),bn属于(-无穷,m),求m取值范围
已知数列{an}的前n项和为Sn.满足an+Sn=2n.设bn=(2-n)(an-2),bn属于(-无穷,m),求m取值范围
已知数列{an}的前n项和为Sn.满足an+Sn=2n.设bn=(2-n)(an-2),bn属于(-无穷,m),求m取值范围
a1=1,
n>=2时,a(n-1)+S(n-1)=2(n-1),
相减得:an-a(n-1) +an=2,
an-2=1/2(a(n-1)-2),an-2=-(1/2)^(n-1),
bn= (n-2)* (1/2)^(n-1),
b(n+1)- bn=(1/2)^n*(3-n),
n>3时,b(n+1)< bn
b3=b4=1/4最大,m