`1/(x^2+9x+20)+1/(x^2+11x+30)+1/(x^2+13x+42)=1/18` (ĐK: `x\ne-4;x\ne-5;x\ne-6;x\ne-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`

`(x+7-x-4)/((x+4)(x+7))=1/18`

`3/((x+4)(x+7))=1/18`

`(x+4)(x+7)=3*18`

`x^2+11x+28=54`

`x^2+11x-26=0`

`(x^2+13x)+(-2x-26)=0`

`x(x+13)-2(x+13)=0`

`(x+13)(x-2)=0`

`x=-13` hoặc `x=2` (Đều thỏa)

vậy: `...`