`\qquad4x(x-5)-(x-1)(4x-3)=5`
`<=> 4x^2-20x-(4x^2-3x-4x+3)=5`
`<=> 4x^2-20x-4x^2+7x-3=5`
`<=> -13x=8`
`<=> x=-8/13`
Vậy `S={-8/13}`
---------------------------------------
`\qquad (3x-4)(x-2)=3x(x-9)-3`
`<=> 3x^2-6x-4x+8=3x^2-27x-3`
`<=> 3x^2-10x+8=3x^2-27x-3`
`<=> 3x^2-10x-3x^2+27x=-3-8`
`<=> 17x=-11`
`<=> x=-11/17`
Vậy `S={-11/17}`
-------------------------------------
`\qquad (x-5)(x-4)-(x-1)(x+1)(x-2)=7`
`<=> x^2-4x-5x+20-(x^2-1)(x-2)=7`
`<=> x^2-9x+20-(x^3-2x^2-x+2)=7`
`<=> x^2-9x+20-x^3+2x^2+x-2=7`
`<=> -x^3+3x^2-8x+18-7=0`
`<=> -x^3+3x^2-8x+11=0`
----------------------------------
`1, (a+b)^2=(a+b)(a+b)=a^2+ab+ba+b^2=a^2+2ab+b^2`(đpcm)
`2, (a-b)^2=(a-b)(a-b)=a^2-ab-ba+b^2=a^2-2ab+b^2`(đpcm)
`3, (a-b)(a+b)=a^2+ab-ba-b^2=a^2-b^2`(đpcm)
