f(x)=4(logax)^2-2logax+b
=(2logax)^2-2logax+b
=(2logax-1/2)^2+b-1/4
f(x)值域为[5,15]
当2logax＝1/2时,x=a^(1/4),b-1/4＝5,b=21/4
当f(x)=15时,(2logax-1/2)^2＝10,2logax-1/2=√10
当a