12/13-（1/2+2/13)
=12/13-2/13-1/2
=10/13-1/2
=20/26-13/26
=7/26
计算：
6/7+1/6-17/42
=36/42+7/42-17/42
=26/42
=13/21
9/10-(7/8-1/2)
=36/40-35/40+20/40
=21/40
3/5+5/9+1/6
=54/90+50/90+15/90
=123/90
=1又33/90
=1又11/30
11/18-(3/1+5/24)
=44/72-3-15/72
=29/72-216/72
=-187/72
=-2又43/72
2/3+3/5+1/10
=20/30+18/30+3/30
=41/30
=1又11/30
1-1/3-2/5
=15/15-5/15-6/15
=4/15
70分钟=(1又1/6)小时
计算下列各题后,你有什么发现?
1/2+1/3=5/6
1/3-1/4=1/12
1/2-1/5=3/10
1/3+1/7==10/21
发现(加法的规律：新分母=两个老分母相乘,新分子=两个老分母相加.减法规律：新分母=两个老分母相乘,新分子=较大的分母-较小的分母 ). 12/13-（1/2+2/13)
=12/13-2/13-1/2
=10/13-1/2
=20/26-13/26
=7/26
计算：
6/7+1/6-17/42
=36/42+7/42-17/42
=26/42
=13/21
9/10-(7/8-1/2)
=36/40-35/40+20/40
=21/40
3/5+5/9+1/6
=54/90+50/90+15/90
=123/90
=1又33/90
=1又11/30
11/18-(3/1+5/24)
=44/72-3-15/72
=29/72-216/72
=-187/72
=-2又43/72
2/3+3/5+1/10
=20/30+18/30+3/30
=41/30
=1又11/30
1-1/3-2/5
=15/15-5/15-6/15
=4/15
70分钟=(1又1/6)小时
计算下列各题后,你有什么发现?
1/2+1/3=5/6
1/3-1/4=1/12
1/2-1/5=3/10
1/3+1/7==10/21
发现(加法的规律：新分母=两个老分母相乘,新分子=两个老分母相加.减法规律：新分母=两个老分母相乘,新分子=较大的分母-较小的分母 ).