已知数列{an}是首项a1>0,
a1=a>0,an=aq^(n-1)。-10。
--->Sn=a(1-q^n)/(1-q)
b1=a2-ka3=aq-kar^2=aq(1-kq);
bn=a(n+1)-ka(n+2)=aq^n-k*aq^(n+1)=aq^n(1-kq)
--->Tn=aq(1-kq)(1-q^n)/(1-q)
Tnaq(1-kq)(1-q^n)/(1-q)0--->-11-q>0;1-q^n>0,a>0
--->q(1-kq)(1+q^2)k>q--->k>q/(1+q^2)
2)q=1:Sn=na;Tn=(1-k)na
Tn(1-k)na1-kk>1/2
3)q>1--->1-qq(1-kq)k>...全部
a1=a>0,an=aq^(n-1)。-10。
--->Sn=a(1-q^n)/(1-q)
b1=a2-ka3=aq-kar^2=aq(1-kq);
bn=a(n+1)-ka(n+2)=aq^n-k*aq^(n+1)=aq^n(1-kq)
--->Tn=aq(1-kq)(1-q^n)/(1-q)
Tnaq(1-kq)(1-q^n)/(1-q)0--->-11-q>0;1-q^n>0,a>0
--->q(1-kq)(1+q^2)k>q--->k>q/(1+q^2)
2)q=1:Sn=na;Tn=(1-k)na
Tn(1-k)na1-kk>1/2
3)q>1--->1-qq(1-kq)k>q/(1+q^2)
q<>1:1+q^2>2q--->q/(1+q^2)-1都有k>1/2。收起
