Giải các phương trình sau:

`a)`

`x^2 -2x(3x+1)=-5(x^2 +1)+21`

`<=>x^2 -6x^2 -2x=-5x^2 -5+21`

`<=>-5x^2 -2x=-5x^2 +16`

`<=>-2x=16`

`<=>x=-8`

Vậy `S={-8}`.

`b)`

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

`<=>x^2 +5x-2x-10=x^2 -6x+9+9x`

`<=>x^2 +3x-10=x^2 +3x+9`

`<=>-10=9` (vô lí)

Vậy `S={∅}`.

`c)`

`2x(2x-7)-9=(2x+3)(2x-3)-14x`

`<=>4x^2 -14x-9=4x^2 -9-14x`

`<=>-9=-9` (luôn đúng)

Vậy `S={x|x\in RR}`.

`d)`

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

`<=>6x^2 +2x-3x-1=9x^2 +12x+4-3x^2`

`<=>6x^2 -x-1=6x^2 +12x+4`

`<=>-13x=5`

`<=>x=-5/13`

Vậy `S={-5/13}`.