|a-b|=|b|,
平方得a^2-2ab=0,a≠0,
∴ab=a^2/2>0,
|2b|^2-|a-2b|^2
=4b^2-(a^2-4ab+4b^2)
=a^2>0,
∴|2b|^2>|a-2b|^2,
∴|2b|>|a-2b|.