Đáp án+Giải thích các bước giải:
`a) A = (x - y)² + (x + y)²`
`= (x² - 2xy + y²) + (x² + 2xy + y^{2})`
`= x^{2} - 2xy + y^{2} + x^{2} + 2xy + y^{2}`
`= 2x² + 2y²`
`b) B = (2a + b)² - (2a - b)²`
`= [(2a + b) - (2a - b)] . [(2a + b) + (2a - b)]`
`= (2a + b - 2a + b).(2a + b + 2a - b)`
`= 2b . 4a`
`= 8ab`
`c) C = (x + y)² - (x - y)²`
`= [(x + y) - (x - y)] . [(x + y) + (x - y)]`
`= (x + y - x + y)(x + y + x - y)`
`= 2y . 2x`
`= 4xy`
`d) D= (2x - 1)² - 2(2x - 3)² + 4`
`= (4x² - 4x + 1) - 2(4x² - 12x + 9) + 4`
`= 4x² - 4x + 1 - 8x² + 24x - 18 + 4`
`= - 4x² + 20x - 13`
