你的答案似乎不對(duì),因?yàn)槲易鲞^(guò)這道題三遍.
1.a1,a3,a5成等比
則：a3^2=a1*a5
又a1,a3,a5是等差數(shù)列{an}中的項(xiàng)
則:a3=a1+2d
a5=a1+4d
則有:(a1+2d)^2=a1(a1+4d)
a1^2+2a1d+4d^2=a1^2+4a1d
4d^2-2a1d=0
2d^2-a1d=0
d(2d-a1)=0
又d不為0
則:2d-a1=0
a1=2d
則(a1+a3+a9)/(a2+a4+a10)
=[a1+a1+2d+a1+8d]/[a1+d+a1+3d+a1+9d]
=[3*2d+10d]/[3*2d+13d]
=16/19
2.an=Sn-S(n-1) (n>=2)
=(1/2)^n+a-[(1/2)^(n-1)+a]
=(1/2)^n-(1/2)^(n-1)
=(1/2)^n-[(1/2)^n]/[(1/2)]
=(1/2)^n-2*[(1/2)^n]
=(1-2)*(1/2)^n
=-(1/2)^n
又a1=S1=(1/2)^1+a
=1/2+a
則:an=-(1/2)^n (n>=2)
=1/2+a (n=1)
又{an}為等比數(shù)列
則:a1=-(1/2)^1=1/2+a
-1/2=1/2+a
則:a=-1