Bài 4:
`a) (x+1/4)^2 = x^2 + 2.x.1/4 + (1/4)^2 = x^2 + 1/2 x + 1/16`

`b) (2x - 1/2)^2 = (2x)^2 - 2.2x.1/2 + (1/2)^2 = 4x^2 - 2x + 1/4`

`c) (x+1/3)^3 = x^3 + 3.x^2 . 1/3 + 3.x.(1/3)^2 + (1/3)^3`

` = x^3 + x^2 + 1/3 x + 1/27`

`d) (3x-1/3)^3`

` = (3x)^3 - 3.(3x)^2.1/3 + 3.3x.(1/3)^2 - (1/3)^3`

` = 27x^3 - 9x^2 + x - 1/27`

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Bài 5:

`a) A = (2x-5)^2 + (2x-7)^2 - 2(2x-5)(2x-7)`

` = [(2x-5) - (2x-7)]^2`

` = 2^2`

`=4`

`b) B = (2x-3)^3 + (2x+3)^3 - 16x^3 + 25x`

` = (2x-3+2x+3)[(2x-3)^2 - (2x-3)(2x+3) + (2x+3)^2] - 16x^3 + 25x`

` = 4x(4x^2 - 12x + 9 - 4x^2 + 9 + 4x^2 + 12x + 9) - 16x^3 + 25x`

` = 4x(4x^2 +27) - 16x^3 + 25x`

` = 16x^3 + 108x - 16x^3 + 25x`

` = 108x + 25x`

` = 133x`