因为an是等比数列，所以a4a5a6/a1a2a3=a7a8a9/a4a5a6所以a4a5a6*a4a5a6=a1a2a3*a7a8a9=5*10=50a4a5a6=√50 = 5√2
(a4a5a6)^2=a1a2a3X(a7a8a9)=5X10=50，所a4a5a9=5V2
5倍根号2a1a2a3a7a8a9=（a4a5a6）^2
