4/5×1/2+2/5÷0.6-1/15=2/5+10/15-1/15=6/15+10/15-1/15=11/1×3＋1／3×5＋1／5×7＋.＋1/17×19＋1/19×21=1/2×(1-1/3+1/3-1/5+1/5-1/7.+1/19-1/21）=1/2×（1-1/20）=10/219999*2222+3333*3334=3333×3×2222+3...