（根号20+根号5）/(根号5-2)+2/(根号5-3)
=3根号5/(根号5-2)+2/(根号5-3)
=3根号5*(根号5+2)/(5-4)+2*(根号5+3)/(5-9)
=15+6根号5-2(根号5+3)/4
=15+6根号5-根号5/2-3/2
=27/2-11根号5/2