9又5/7-(2又1/13+4又5/7+11/13)
=9又5/7-2又1/13-4又5/7-11/13
=9又5/7-4又5/7-(11/13+2又1/13)
=5-2又12/13
=2又1/13
这样的题,要先用交换律、结合律,先算同分母相加减,