1. x2 - 4y2 2. 25a2 - b2 3. 36x2 - 9y2 4. 16x2y2 - 64y2

1. x^2 - 4y^2 = (x + 2)(x - 2)

2. 25a^2 - b^2 = (5a + b)(5a - b)

3. 36x^2 - 9y^2 = (6x + 3y)(6x - 3y)

4. 16x^2y^2 - 64y^2 = (4xy + 8y)(4xy - 8y)