[2xy/（x·根号y+y·根号x）]=2根号xy的平方/[根号xy(根号x+根号y)]
=2根号xy/(根号x+根号y)
[x+y/(根号x+根号y)]+2根号xy/(根号x+根号y)=
[(x+y+2根号xy)/(根号x+根号y)]=(根号x+根号y)的平方/(根号x+根号y)]
=(根号x+根号y)]