你写反了,呵呵.
3/7-1/3+4/7-2/3
=(3/7+4/7)-(1/3+2/3)
=1-1
=0
3/12+12/13-7/13+11/12
=(3/12+11/12)+(12/13-7/13)
=14/12+5/13
=7/6+5/13
=91/78+30/78
=121/78