I am in the first item? +2mn=(m+n)？ -n？ =(m+n+n)*(m+n-n)=(m+2n)*m, so the original formula =n/2m- 1/2*(m+2n).

The second term 2n? -mn=n(2n-m), then the original formula can be changed to-1/n+2/(2n-m).

Similarly, the denominator of the third term can be changed to (m+2n)(m-2n), and then the formula can finally be changed to n/2*(m+2n)+n/2*(m-2n).