因为a1 a3 a9成等比数列
所以a3^2=a1*a9
(a1+2d)^2=a1(a1+8d)
化简得a1=d 代入下式中
a1+a3+a9/a2+a4+a10 = (3a1+10d)/(3a1+13d)=13/16