1. Add 2 mn, -4 mn, 8 mn, -6 mn

2. Add a + b - 1, b - a + 1, 1 - 26

3. Add 4x²y, -3x²y, -7xy², 7xy²

4. Simplify : p + (p - q) + q + (q - p)

5. Simplify : z² + 11z² - 5z - 11z² + 5z

6. Subtract - xy from xy

7. Subtract y² from - 5y²

8. What should be added to x² + y² to get x² + y² + 2 xy?

9. What should be subtracted from x² + y² - 2xy to get x² + y²?

10. Find the value of the expression x + 2 for x = -2