(1) when a is positive and b is negative.

The original formula =-a/a+b/b+(-ab/ab) =-1+1+(-1) =-1.

② When A is negative, B is positive.

The original formula = a/a+(b/-b)+(-ab/ab) =1+(-1)+(-1) =-1.

From ① ②: the value is-1.

(2)m+n=0，ab= 1

Original formula =m+n/a+b- 1/ab.

=0÷ 1- 1÷ 1

=0- 1

=- 1