1/2+1/3=5/6;1/2-1/3=1/6;
1/2+1/4=3/4;1/2-1/4=1/4;
1/2+1/5=7/10;1/2-1/5=3/10;
1/2+1/6=2/3;1/2-1/6=1/3;
1/3+1/4=7/12;1/2-1/4=1/12;
1/3+1/5=8/15;1/3-1/5=2/15;
1/3+1/6=1/2;1/3-1/6=1/6;
1/4+1/5=9/20;1/4-1/5=1/20;
1/4+1/6=5/12;1/4-1/6=1/12;
1/5+1/6=11/30;1/5-1/6=1/30;