1/7+2/3-1/2=.....1/2+1/4+1/8=.....5/12+5/6-3/4=.....5/8+1/10-7/25=....17/60-19/80+21/40=...21/33+13/44-26/55=...

1/7+2/3-1/2=6/42+28/42-21/42=13/42; 1/2+1/4+1/8=4/8+2/8+1/8=7/8; 5/12+5/6-3/4=5/12+10/12-9/12=6/12=1/2; 5/8+1/10-7/25=125/200+20/200-56/200=89/200; 17/60-19/80+21/40=51/240-57/270+126/240=120/240=1/2; 21/33+13/44-26/55=420/660+195/660-312/660=303/660=101/220