9. 8x3y2 – 7x2y + 8x + 4- 6x3y2 + 2x²y + 4x²y – 3x + 5
10. 3r3+ + 5rt? – 6rt + 5+ 4rt - 3+ 6rt?
11. 8x* - 7x3 + 4x4 + 2y3 – 7xy + 3y3 + 12xy
12. 3a3b + 4a²b2 – 7a3b + 2ab - 5a²b2 + 10a3b
13. 5y5 + 4y+ + 3y 3 – 6y2 + 8 - 7y3 + 11 – 6y3
14. 10c2d2 + 6cd3 - 3cd +5c2d2 – 4cd3 + 3c2d?
15. 4x3 + 3x²y + 2xy? – 6y3 + 5x3 – 25y3 + 3xy?
16. (3x2 + 2xy + 4y2) + (6x2 – 5xy + 3y?) + (9x2 – 25y?)
17. (4a3 + 7a²b + 663) + (a3 + 2ab + 463)
18. (2x2 – 3x + 6) + (5x2 – 9x + 4) + (8x2 + 12x)
19. (3x2 - 4x + 7) + (3x – 3x?) + (x – 7)
20. (12ab2 – 7a²b) + (3ab + 4a²b + 6ab)