a2+3a4+5a7/a4+3a【6】+5a9
=a1(q+3q^3+5q^6)/ a1(q^3+3q^5+5q^8)
=q(1+3q^2+5q^5)/ q^3(1+3q^2+5q^5)
=1/q^2
=1/4