解；设首相为a
则a1=a,a3=a+2d,a9=a+8d,a2=a+d,a4=a+3d,a10=a+9d
a1,a3,a9成等比数列
(a+2d)^2=a*(a+8d)
a=d
所以（a1+a3+a9）/（a2+a4+a10）＝(a+a+2d+a+8d)/(a+d+a+3d+a+9d)
=13a/16a=13/16