1) (m+n)+(m-n)=m+n+m-n=2m (m+n)-(m-n)=m+n-m+n=2n
3) (a+2ab+b)+(-4x+5x-4)=a+2ab+b-4x+5x-4=a+2ab+b+x-4 (a+2ab+b)-(-4x+5x-4)=a+2ab+b+4x-5x+4=a+2ab+b-x+4
2) (a+2ab+b)+(a+ 2ab+ b)=a+2ab+b+a+2ab+b=2a+4ab+2b (a+2ab+b)-(a+ 2ab+ b)=a+2ab+b-a-2ab-b=0
4) (5x - 5x+4)+(-4x +5x-4)=5x-5x+4-4x+5x-4=9x (5x - 5x+4)-(-4x +5x-4)=
=5x-5x+4+4x-5x+4=-x+8