Đáp án+Giải thích các bước giải:
`a)`
`3/(4(x+5))+15/(2(25-x^2))=(-7)/(6(x+5))(ĐK:x\ne+-5)`
`↔3/(4(x+5))+(-15)/(2(x^2-25))=(-7)/(6(x+5))`
`↔(3.3.(x-5)-15.6)/(12(x^2-25))=(-7.2.(x-5))/(12(x^2-25))`
`→9x-45-90=-14x+70`
`↔9x-135=-14x+70`
`↔9x+14x=70+135`
`↔23x=205`
`↔x=205/23(TM)`
Vậy `S={205/23}`
`b)`
`x^4+x^3+x+1=0`
`↔x^3(x+1)+(x+1)=0`
`↔(x^3+1)(x+1)+0`
`↔` \(\left[ \begin{array}{l}x^3+1=0\\x+1=0\end{array} \right.\) `↔` \(\left[ \begin{array}{l}x=-1\\x=-1\end{array} \right.\)
Vậy `S={-1}`