`8)1/(x^{2}+9x+20)+1/(x^{2}+11x+30)+1/(x^{2}+13x+42)=1/18`
`<=>1/((x+4)(x+5))+1/((x+5)(x+6))+1/((x+6)(x+7))=0`
`đk:x ne -4,-5,-6,-7`
`pt<=>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`
`<=>(x+7-x-4)/((x+4)(x+7))=1/18`
`<=>3/(x^{2}+11x+28)=1/18`
`<=>x^{2}+11x+28=54`
`<=>x^{2}+11x-26=0`
`<=>x^{2}-2x+13x-26=0`
`<=>x(x-2)+13(x-2)=0`
`<=>(x-2)(x+13)=0`
\(\Leftrightarrow\left[ \begin{array}{l}x=2\\x=-13\end{array} \right.\)
Vậy `S={2,-13}`
