2*sqrt(b/a)-b*sqrt(a/b)
=2*sqrt(b/a)+b*sqrt(b/a)
=(2+b)*sqrt(b/a)
=(2+b)*sh(b/ac)/*sh()是双曲正弦函数*/
=(2b)*sh(b/ac)*(2+b)/2b
=1*2b
=2B