因为为等比数列a10*a11=a1*q^9*a1*q^10=a1*a20log(1/2)a1+log(1/2)a2+....+log(1/2)a20=log(1/2)(a1*a2*....*a20)等比数列性质a1*a20=a2*a19=....=a10*a11(共十项）所以上式=log(1/2)(4^10)=-20
