Tìm x: a) 4x^2 + 4x + 1 = 0; b) x^2 - 2x + 1 = 0; c) 9 - 6x + x^2 = 0; d) -x^2 + 4x - 4 = 0

a) 4x² + 4x + 1 = 0
b) x² - 2x + 1 = 0
c) 9 - 6x + x² = 0
d) -x² + 4x - 4 = 0