1/3+1/6+1/2
=1/3+1/2-1/3+1/2
=1/2+1/2
=1
一又4/5-7/20-9/20
=1+4/5-(7/20+9/20)
=1+4/5-16/20
=1+4/5-4/5
=1
17/20-（3/8+1/5）
=17/20-(15/40+8/40)
=17/20-23/40
=34/40-23/40
=11/40
13/17-（5/17+1/10）+0.9
=13/17-5/17-1/10+9/10
=8/17+8/10
=8/17+4/5
=40/85+68/85
=108/85