1)
x² - 4xy + 4y²
= x² - 2.x.2y + (2y)²
= (x - 2y)²
2)
9x² - y²
= (3x)² - y²
= (3x - y)(3x + y)
3)
9x²y² - 6xy + 1
= (3xy)² - 2.3xy.1 + 1²
= (3xy - 1)²
4)
x² - (y² - 4y + 4)
= x² - (y² - 2.y.2 + 2²)
= x² - (y - 2)²
= [x - (y - 2)][x + (y - 2)]
= (x - y + 2)(x + y - 2)