a2=a1*q,a3=a1*q^2,故有a1+a1*q=6,a1*q+a1*q^2=3,解得q=1/2，a1=4；故a3+a4+a5+a6=a1*q^2+a1*q^3+a1*q^4+a1*q^5=15/8
答案是15/8
