①3/a+(a-15)/5a
=［15+a-15]/5a
=a/5a
=1/5
②(m+2n)/(n-m)+n/(m-n)-2n/(n-m)
=(m+2n)/(n-m)-n/(n-m)-2n/(n-m) ——化为同分母,
=(m-n)/(n-m)
=-1.