6. B

7. A

D. warm

sai chủ đề rùi

copy cat >:(

a) A(x) = 2x³ + 5 + x² - 3x - 5x³ - 4

            = 2x³ - 5x³  + x² - 3x + 5 - 4

            = -3x³ + x² - 3x + 1

    B(x) = -3x⁴ - x³ + 2x² + 2x + x⁴ - 4 - x²

            = -3x⁴ + x⁴ - x³ + 2x² - x² + 2x - 4

            = -2x⁴ - x³ + x² + 2x - 4

b) 

H(x) = A(x) - B(x)

H(x) = (-3x³ + x² - 3x + 1) - (-2x⁴ - x³ + x² + 2x - 4)

        = -3x³ + x² - 3x + 1 + 2x⁴ + x³ - x² - 2x + 4

        = 2x⁴ - 3x³ + x³ + x² - x²  - 3x - 2x + 1 + 4

        = 2x⁴ - 2x³ -5x + 5

1. If we plant more trees, our neighborgood will be greener.

2. If we cycle to school everyday, we will keep the air clean and fresh.

3. If we keep hunting animals, they will be disappeared soon.

1. Lan cycles skillfully.

2. Their children are very good students.

3. Why don't we go to the cinema?

4. How about visiting the musem this afternoon?

5. What about staying at home and do our homework?