`a)`

`A=(3x-4)(x+1)-(2x-3)(x-4)-(x^2-x+1)`

`A=3x^2+3x-4x-4-(2x^2-8x-3x+12)-x^2+x-1`

`A=3x^2-x-4-(2x^2-11x+12)-x^2+x-1`

`A=3x^2-x-4-2x^2+11x-12-x^2+x-1`

`A=(3x^2-2x^2-x^2)+(-x+11x+x)+(-4-12-1)`

`A=11x-17`

Thay `x=1/2`, ta có:

`A=11·1/2-17=11/2-17=-23/2`

`b)`

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

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

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

`B=x^3-x^2-2x-3x^2+3x+6-x^3-3x^2+x-9`

`B=(x^3-x^3)+(-x^2-3x^2-3x^2)+(-2x+3x+x)+(6-9)`

`B=-7x^2+2x-3`

Thay `x=-1/2`, ta có:

`B=-7·(-1/2)^2+2·(-1/2)-3`

`B=-7·1/4-1-3`

`B=-7/4-4`

`B=-23/4`

`c)`

`C=3(x-5)(x+5)-(x+1)(x^2-x+1)+(x-1)(x^2+x+1)`

`C=3(x^2-25)-(x^3+1)+x^3-1`

`C=3x^2-75-x^3-1+x^3-1`

`C=(-x^3+x^3)+3x^2+(-75-1-1)`

`C=3x^2-77`

Thay `x=-2`, ta có:

`C=3·(-2)^2-77=3·4-77=12-77=-75`

`d)`

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

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

`D=6·2x+3x^2-6x+3`

`D=6x+3x^2-6x+3`

`D=3x^2+3`

Thay `x=1/3`, ta có:

`D=3·(1/3)^2+3=3·1/9+3=1/3+3=10/3`