`#DyHungg`
`1/(x²+9x+20)+1/(x²+11x+30)+1/(x²+13x+42)=1/18`
`ĐKXĐ: x` $\neq$ `4;5;6;7`
`⇔1/(x²+4x+5x+20)+1/(x²+5x+6x+30)+1/(x²+6x+7x+42)=1/18`
`⇔1/(x(x+4)+5(x+4))+1/(x(x+5)+6(x+5))+1/(x(x+6)+7(x+6))=1/18`
`⇔1((x+4)(x+5))+1/((x+5)(x+6))+1/((x+6)(x+7))=1/18`
`⇔1/(x+4)-1/(x+5)+1/(x+5)-1/(x+6)+1/(x+6)-1/(x+7)=1/18`
`⇔1/(x+4)-1/(x+7)=1/18`
`⇔(18(x+7))/(18(x+4)(x+7))-(18(x+4))/(18(x+4)(x+7))=((x+4)(x+7))/(18(x+4)(x+7))`
`⇒18x-126-18x+72=x²+11x-28`
`⇔x²+11-26=0`
`⇔x²+13x-2x-26=0`
`⇔x(x+13)-2(x+13)=0`
`⇔(x+13)(x-2)=0`
`1) x+13=0⇔x=-13`
`2) x-2=0⇔x=2`
Vậy `S={2;-13}`
