sai gồi pro ạ :-:

thiếu đề 

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

`=x^4 +6x^2 +9-x^4 -3x^2 -3(x^2-1)`

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

`=12`

`=>bbD`

`(3x-2y)^2`

`=(3x)^2-2.3x.2y+(2y)^2`

`=9x^2-12xy+4y^2`

https://www.facebook.com/HoidapVietJack/photos/pcb.283511710491373/283511090491435

`ĐKXĐ:x^2-4x+4>=0`

`<=>(x-2)^2>=0(luôn đúg )`

Vậy đkxđ `AAx`

`(a+b+c)^2-2(a+b+c)(b+c)+(b+c)^2`

`=(a+b+c-b-c)^2`

`=a^2`

Áp dụng hđt2

`5(x-3)(x-7)-(5x+1)(x-2)=8`

`<=>5(x^2 -3x-7x+21)-(5x^2+x-10x-2)=8`

`<=>5x^2 -50x+105-5x^2 +9x+2=8`

`<=>-41x+107=8`

`<=>107-8=41x`

`<=>41x=99`

`<=>x=99/41`

Vậy `S={99/41}`