Bài `2:`
`a, ( 2x - 3y )^{2} = 4x^{2} - 12xy + 9y^{2}`
`b, ( 5x - y )^{2} = 25x^{2} - 10xy + y^{2}`
`c, ( 2x + y^{2} )^{2} = 4x^{2} - 4xy^{2} + (y^{2})^{2}`
`d, ( x^{2} + \frac{2}{5}y )( x^{2} - \frac{2}{5}y ) = x^{2} - (\frac{2}{5}y)^{2}`
`e, ( x + \frac{1}{4} )^{2} = x^{2} + \frac{1}{2}x + \frac{1}{16}`
`f, ( \frac{2}{3}x^{2} - \frac{1}{2} )^{2} = \frac{4}{9}x^{4} - \frac{2}{3}x^{2} + \frac{1}{4}`
`k, ( x + 2y + z )( x + 2y -z ) = ( x + 2y )^{2} - z^{2}`
Bài `3 : `
`a, (` 3x `- 2x )( 3x +` 2x `) = 9x^{2} + 4`
`b, ( 2x + 1 )^{2} =` 4`x^{2}` `+ 4x + 1`
`c, ( 2x - 1 )^{2} = 4x^{2} -` 4x `+ 1`
`d, 9 -` 16`x^{2}` `= ( 3 - 4x )( 3 + 4x )`
`e, ( 4x^{2} - 3 ) = ( 2x -` √3 `)(` 2x `+ √3 )`
