过程如下：首项=a，公比=qa1a2a3=5a^3q^3=5a7a8a9=20a^3*q^(6+7+8)=a^3*q^21=20相除q^18=4q^54=(a^18)^3=64a19a20a21=a^3*q^(18+19+20)=a^3*q^57=a^3q^3*q^54=5*64=320
