a) a^4 - 4a^2 + 4 = (a^2-2)^2
b) y^2+9+6y = y^2 + 6y + 9 = (y+3)^2
c) x^2 - 2 rad 2 x + 2 = (x-rad 2)^2
d) (a-2)^2 + 2(a-2)(a+1) + (a+1)^2 = [(a-2)+(a+1)]^2 = (2a-1)^2
e) 81x^4 - 16 = (9x^2-4)(9x^2+4)
f) 49a^2-1 = (7a-1)(7a+1)
g) 4x^2y^2-25x^2 = (2xy-5x)(2xy+5x)
sau 4x^2y^2-25x^2 = x^2(4y^2-25) = x^2(2x-5)(2y+5)
h) a^2-4/9 = (a-2/3)(a+2/3)
g) 16/25 - b^4 = (4/5-b^2)(4/5+b^2)