1) m+n + ( m+n) = m + n+ m-n =2m
2)( a^2+2ab+b^2) + ( a^2-2ab+b^2) =
=a^2+2ab+b^2 + a^2-2ab+b^2 = 2a^2 + 2b^2
3) (a^2+2ab+b^2) + (-a^2+2ab-b^2)=
=a^2+2ab+b^2 -a^2+2ab-b^2 = 4ab
4) 5x^2-5x+4 +(-4x^2+5x-4) = 5x^2-5x+4 - 4x^2+5x-4=x^2
5)a^2-a+1 -( a+1) =a^2- a+1- a -1 =a^2 -2a
6)x^2+xy+y^2 -( x^2-xy+y^2) = x^2+xy+y^2 - x^2+xy-y^2=
=2xy
7)3x+7y -( 2x-5y) = 3x+7y - 2x+5y= x +12y
8)8a+9b-10c - ( 9a-10b+11c) =8a+9b-10c - 9a+10b - 11c =
=-a + 19b -21c