分数加减混合运算,下面各题怎样简便就怎样算.（递等式）

(1)5/6-4/9+1/6-2/9
=(5/6+1/6)-(4/9+2/9)
=1-2/3
=1/3
(2)5/8+1/8-3/10-1/10
=(5/8+1/8)-(3/10+1/10)
=3/4-2/5
=15/20-8/20
=7/20
(3)9/10-(1/10+2/5)
=9/10-1/10-2/5
=4/5-2/5
=2/5