设{an}为等比数列,公比为q.已知a1+a3=10,a4+a6=5\4,求an与sn.（2）已知a1*a9=64,a3+a7=20,求a11.

设{an}为等比数列,公比为q.已知a1+a3=10,a4+a6=5\4,求an与sn.（2）已知a1*a9=64,a3+a7=20,求a11.
设{an}为等比数列,公比为q.已知a1+a3=10,a4+a6=5\4,求an与sn.
（2）已知a1*a9=64,a3+a7=20,求a11.

设{an}为等比数列,公比为q.已知a1+a3=10,a4+a6=5\4,求an与sn.（2）已知a1*a9=64,a3+a7=20,求a11.
(a1+a3)q*q*q=a4+a6;
即10*q*q*q=5/4;
q=1/2;
a1+a3=a1(1+q*q)=10;即a1（1+1/4)=10;a1=8;
an=a1*q^(n-1)=8*(1/2)^(n-1)=(1/2)^(n-4);
Sn=[1-(1/2)^n]/16