我先占个位
解
1:即94/3*83/2-34/3*83./2-83/2*20
=(94/3-34/3-20)*83/2
=0
2:7/12-9/20+11/30-13/42+15/56-17/72+19/90
=1/3+1/4-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)+(1/7+1/8)-(1/8+1/9)+(1/9+1/10)
=1/3+1/10=13/30
我先占个位
解
1:即94/3*83/2-34/3*83./2-83/2*20
=(94/3-34/3-20)*83/2
=0
2:7/12-9/20+11/30-13/42+15/56-17/72+19/90
=1/3+1/4-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)+(1/7+1/8)-(1/8+1/9)+(1/9+1/10)
=1/3+1/10=13/30