1/5+1/4。=4/20+5/20=(4+5)/20=9/201/5-1/6。=6/30-5/30=(6-5)/30=1/301/4-1/7。=7/28-4/28=(7-4)/28=3/281/3-1/8。=8/24-3/24=(8-3)/24=5/245/12+3/4+1/12=5/12+9/12+1/12=(5+9+1)/12=15/12=5/4