1) (x + y) + (y + x) = x + y + y + x = 2x + 2y;
2) (a - b) - (a - b) = a - b - a + b = a - a - b + b = 0;
3) (c - d) - (c + d) = c - d - c - d = c - c - d - d = -2d.
1) (x + y) + (y + x) = x + y + y + x = 2x + 2y;
2) (a - b) - (a - b) = a - b - a + b = a - a - b + b = 0;
3) (c - d) - (c + d) = c - d - c - d = c - c - d - d = -2d.