等比数列{an)中,有：a(n-1)a(n+1)=an^2
a5*a6*a7=a6^3=3,a6*a7*a8=a7^3=24
a6*a8=a7^2
a8=a7^2/a6
a7a8a9=a8^3
=(a7^2)^3/a6^3
=a7^6/a6^3
=24^2/3=
8*24
=192