(1)a1*a2*.*a18=2^18
a1(a1q)*.*(a1q^17)=2^18
a1^18*q^(1+2+...+17)=2^18
a1^18*q^(17*9)=2^18
a1^9*a1^9*(q^17)^9=2^18
a1^9*(a1q^17)^9=2^18
a1^9*a18^9=2^18-------------------(a1*a18)^9=4^9
(a1*a1*q^17)^9=4^9
(a5*a5*q^9)^9=4^9
a5*a5*q^9=4 (1)
a5+a5*q^9=5 (2)
a5+4/a5=5
解得a5=1或者2
a5=1,代入（1）,q=2的立方根,
a5=2,代入（1）,q=1.
(2)q=2,
a3a6a9.a18
=(a1q^2)*(a1*q^5)...(a1*q^17)
=a1^6*q^(2+5+..+17)
=a1^6*q^57
=(a1*a18)^3*q^6 由上面推知 (a1*a18)^9=4^9---》a1*a18=4
=4^3*2^6
=2^12