Ответ:

  1. \(x^2y+xy^2\).
  2. \(18xy^2-35x^2y\).
  3. \(6a^3b+8a^2c\).
  4. \(-120xy^2+28x^2y\).
  5. \((11-8+3+1)a^5=7a^5\).
  6. \((1.9-2.9-1)x^3=-2x^3\).
  7. \((20+5-17)xy=8xy\).
  8. \((8-3+1-7)ab^2=-ab^2\).