`a) 25x^6 - (4y^2)/49`
`= 5^2 (x^3)^2 - (2^2y^2)/7^2`
`= (5x^3)^2 - (2y)^2/7^2`
`= (5x^3)^2 - ((2y)/7)^2`
`= (5x^3 + (2y)/7)(5x^3 - (2y)/7)`
`b) (3x + 2)^2 - 4`
`= (3x + 2)^2 - 2^2`
`= (3x + 2 - 2)(3x + 2 + 2)`
`= 3x(3x + 4)`
Áp dụng: `x^2 - y^2 = (x + y)(x - y)`