a1=a2/q;
a3=a2q
a1+a2+a3=7
a1a2a3=8
a1a2a3=a2^3
那么a2=2
2/q+2+2q=7
2q^2-5q+2=0
q=2或者q=1/2
an=2^(n-1)或者1/2^(n-3)