用简便方法计算各题分数的题
1/5+5/6+4/5
=（1/5+4/5）+5/6
=1+5/6
=1又5/6
7/9-1/7-2/9
=（7/9-2/9）-1/7
=5/9-1/7
=25/63
2/15+4/21+7/15+2/21
=（2/15+7/15）+（4/21+2/21）
=3/5+2/7
=31/35
2/3-1/2+2/3-1/2
=（2/3+2/3）-（1/2+1/2）
=4/3-1
=1/3
5/12+（1/12-2/5）
=（5/12+1/12）-2/5
=1/2-2/5
=1/10
2/15+（2/5-1/15）-1/5
=（2/15-1/15）+（2/5-1/5）
=1/15+1/5
=4/15