`a, 5x(12+7) - 2x(20x-5) = -100`
`<=>95x-40x^2+10x=-100`
`<=>105x-40x^2=-100`
`<=>15x-40x^2+100=0`
`<=>x=(21-√1081)/16` hoặc `x=(21+√1081)/16`
Vậy `S={(21-√1081)/16 ; (21+√1081)/16}`
`b, 0,6x(x-0,5) - 0,3x(2x+1,3) = 0,138`
`<=>-0,69x=0,138`
`<=>x=0,2`
Vậy `S={0,2}`
`c; 5x.(2x-4) - 2x.(5x-6) = x.(x-8) -36`
`<=>10x^2-20x-10x^2+12x=x^2-8x-36`
`<=>36=x^2`
`<=>=x+-6`
Vậy `S={+-6}`