log2(b)*loga(2)=ln(b)/lb(2)*ln(2)/ln(a)=ln(b)/ln(a)=loga(b)
x^2-[log2(b)+loga(2)]x+loga(b)=0
(x-log2(b))(x-loga(2))=0
x1=log2(b),x2=loga(2)
当x1=-1,x2=2时
b=1/2,a=根号2
当x1=2,x2=-1时
b=4,a=1/2