`1/(x^2+9x+20)+1/(x^2+11x+30)+1/(x^2+13x+72)=1/18`
`đkxđ;x ne -4,-5,-6,-7`
`<=>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+4)(x+7)`
`<=>54=x^2+11x+28`
`<=>x^2+11x-26=0`
`<=>x^2-2x+13x-26=0`
`<=>x(x-2)+13(x-2)=0`
`<=>(x-2)(x+13)=0`
`<=>` $\left[ \begin{array}{l}x=2\\x=-13\end{array} \right.$
Vậy `S={2,-13}`
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