`\qquad 3x(4/3x+1)-4x(x-2)=10`
`<=> 4x^2+3x-4x^2+8x=10`
`<=> 11x=10`
`<=> x=10/11`
Vậy `S={10/11}`
------------------------------------
`\qquad4x(x-5)-7x(x-4)+3x^2=12`
`<=>4x^2-20x-7x^2+28x+3x^2=12`
`<=> 8x=12`
`<=> x=12/8=3/2`
Vậy `S={3/2}`
------------------------------------
`\qquad (3x+5)(2x-7)`
`=6x^2-21x+10x-35`
`=6x^2-11x-35`
-----------------------------------
`\qquad (-5x+2)(-3x-4)`
`=15x^2+20x-6x-8`
`=15x^2+14x-8`
------------------------------------
`\qquad 4x^2-(x+3)(x-5)+x`
`=4x^2-(x^2-5x+3x-15)+x`
`=4x^2-(x^2-2x-15)+x`
`=4x^2-x^2+2x+15+x`
`=3x^2+3x+15`
