`a, 120/x - 1 = (120-x)/(x+10) + 2/5`
ĐKXĐ : `x \ne 0 , x \ne - 10`
`⇔ (120.5(x+10))/(5x(x+10)) - 1.5x(x+10) = (120-x.5x)/(5x(x+10)) + (2x(x+10))/(5x(x+10))`
`⇒ 600(x+10) - 5x(x+10) = 5x(120-x) + 2x(x+10)`
`⇔ -5x^2 + 550x + 6000 = -3x^2 + 620x`
`⇔ -5x^2 - 70x + 6000 = -3x^2`
`⇔ -2x^2 - 70x + 6000 = 0`
`⇔ -2(x-40)(x+75) = 0`
`⇔`\(\left[ \begin{array}{l}x-40=0\\x+75=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=40(TM)\\x=-75(TM)\end{array} \right.\)
Vậy `S ={40,75}`
`b, (2x-4)(x^2+1) > 0`
`⇔ x^3 - 2x^2 + x - 2 > 0`
`⇔ (x-2)(x^2+1) > 0`
Vì `x^2 + 1 > 0`
`⇔ x - 2 > 0`
`⇔ x > 2`
Vậy `S = {x|x > 2}`
