设公比为q.
a1×a2×a3=(a2/q)×a2×(a2q)=a2³=1
a2=1
a4/a2=q²=(1/9)/1=1/9
q=1/3或q=-1/3
q=1/3时,
a1=a2/q=1/(1/3)=3
S5=a1(1-q^5)/(1-q)
=3×(1-1/3^5)/(1-1/3)
=(9/2)×(1-1/243)
=121/27
q=-1/3时,
a1=a2/q=1/(-1/3)=-3
S5=a1(1-q^5)/(1-q)
=(-3)×[1-(-1/3^5)]/[1-(-1/3)]
=(-9/4)×(1+1/243)
=(-9/4)×(244/243)
=-61/27
综上,得：首项为3,公比为1/3,前5项和为121/27；或首项为-3,公比为-1/3,前5项和为-61/27.