`a)`

`(x-1)(x+1)=x^2 -1^2 =x^2 -1`

`b)`

`(2x-1)(2x+1)=(2x)^2 -1^2 = 4x^2 -1`

`c)`

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

`d)`

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

`e)`

`(5a-3b)(5a+3b)=(5a)^2 -(3b)^2 = 25a^2 - 9b^2`

`f)`

`(3x+1)(3x-1)=(3x)^2 -1^2 =9x^2 -1`

`g)`

`(5x^2 -2)(5x^2 +2) = (5x^2)^2 -2^2 = 25x^4 -4`

`h)`

`(2a + 1/2)(2a - 1/2) = (2a)^2 - (1/2)^2 = 4a^2 - 1/4`

`i)`

`(3x^2 -y)(3x^2 +y) =(3x^2)^2 - y^2 = 9x^4 -y^2`

`j)`

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

`k)`

`(5x - 3/2)(5x + 3/2) = (5x)^2 - (3/2)^2 = 25x^2 - 9/4`

`l)`

`(2a^2 -7)(2a^2 +7) = (2a^2)^2 -7^2 = 4a^4 -49`.