等差数列{an}
a3=a1+2d=5,
a10=a1+9d=-2
d=-1,a1=7
an=8-n
(2)
等比数列{bn}设公比为q,q>0
b1=a7=1
2b3=2q^2=a2+a6=6+2=8
q^2=4, q=2
Sn=b1(2^n-1)/(2-1)=2^n-1
等差数列{an}
a3=a1+2d=5,
a10=a1+9d=-2
d=-1,a1=7
an=8-n
(2)
等比数列{bn}设公比为q,q>0
b1=a7=1
2b3=2q^2=a2+a6=6+2=8
q^2=4, q=2
Sn=b1(2^n-1)/(2-1)=2^n-1