Đáp án:
`a, x² + 2x +1 ( HDT 1)`
`= x² + 2.x.1 + 1²`
`= (x + 1)²`
`b, 9x² + 6xy + y² ( HDT 1)`
`= (3x)² + 2.3x.y + y²`
`= (3x + y)²`
`c, 4a² - 20ab + 25b² ( HDT 2 )`
`= (2a)² - 2.2a.5b + (5b)²`
`= (2a - 5b)²`
`d, x² - x + 1/4 ( HDT 2 )`
`= x² - 2.x. 1/2 + (1/2)²`
`= (x - 1/2)²`
`e, 9x² - 16y² ( HDT 3 )`
`= (3x)² - (4y)²`
`= ( 3x - 4y )(3x + 4y)`
`f, 1/4 x² - 1/9 y² ( HDT 3 )`
`= (1/2 x)² - (1/3 y)²`
`= ( 1/2 x - 1/3 y )( 1/2 x + 1/3 y)`
