5/2+9/4-5/1+9/1 =(2/5-1/5)+(4/9+1/9)=1/5+5/9=9/45+25/45=34/45;15/3+6/5+15/2 =(3/15+2/15)+5/6=5/15+5/6=1/3+5/6=2/6+5/6=7/6;11/5+2/1-11/4 =(5/11-4/11)+1/2=1/11+1/2=2/22+11/22=13/22;21/16-16/3-16/1=16/21...