Đáp án:
e) `4x^2-4x+1`
`= (2x)^2-2.2x.1+1`
`= (2x-1)^2`
f) `9-12x+4x^2`
`= 3^2-2.3.2x+(2x)^2`
`= (3-2x)^2`
g) `x^2-x+1/4`
`= x^2-2.x.1/2+(1/2)^2`
`= (x-1/2)^2`
k) `4a^2x^2+b^2-4abx`
`= (2ax)^2-2.2ax.b+b^2`
`= (2ax-b)^2`
l) `(a^2)/9-2/15 ab+(b^2)/25`
`= (a/3)^2-2.a/3.b/5+(b/5)^2`
`= (a/3-b/5)^2`