（1）a-(-b)-c
=a+b-c
=2又2/3-3又1/2-1又3/4
=2又2/3-5又1/4
=-2又7/12
(2)a-b-c
=2又2/3+3又1/2-1又3/4
=6又1/6+1又3/4
=7又11/12
(3)a-b-(-c)
=a-b+c
=2又2/3+3又1/2-1又3/4
=5又7/6-1又3/4
=4又5/12
(4)b-a-(-c)
=b-a+c
=-3又1/2-2又2/3-1又3/4
=-6又1/6-1又3/4
=-7又11/12
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