a/ $x^2+6x+9\\=x^2+2.x.3+3^2\\=(x+3)^2$
Vậy $x^2+6x+9=(x+3)^2$
b/ $x^2+x+\dfrac{1}{4}\\=x^2+2.x.\dfrac{1}{2}+\left(\dfrac{1}{2}\right)^2\\=\left(x+\dfrac{1}{2}\right)^2$
Vậy $x^2+x+\dfrac{1}{4}=\left(x+\dfrac{1}{2}\right)^2$
c/ $2xy^2+x^2y^4+1\\=(xy^2)^2+2.xy^2.1+1^2\\=(xy^2+1)^2$
Vậy $2xy^2+x^2y^4+1=(xy^2+1)^2$
