3:2
{U/[R1+R2]}*R1=10 ①
{U/[R1+R3]}*R1=8 ②
①/②得[R1+R3]/[R1+R2]=5/4,将R2=1/2R3代入上式,解得R1=3/2R3,即R1:R3=3:2