Câu 1:
a.-8/18-15/27=-4/9-5/9=-1.
b.-3/2:9/4=-3/2.4/9=-2/3.
c.1/14.9/14+5/14.1/14-15/14
=1/14.9/14+5/14.1/14-15.1/14
=1/14.(9/14+5/14-15)
=1/14.(-14)
=-1.
d.B=2/11.13+2/13.15+2/15.17+...+2/53.55
=1/11-1/13+1/13-1/15+1/15-1/17+...+1/53-1/55
=1/11-1/55
=4/55.