Replace ∗ with a monomial so that the result is an identity:

1. (2a + ∗)(2a - ∗) = 4a^2–b^2

2. (∗− 3x )(∗ +3x) = 16y^2–9x^2

3. 100m^4–4n^6 = (10m^2−∗)(∗ +10m^2)

4. m^4–225c^10 = (m^2−∗)(∗ +m^2).