Câu 4:
-ĐKXĐ: \(x\notin\left\{-1;-3;-5;-7;-9\right\}\).
PT \(\Leftrightarrow\dfrac{1}{\left(x+1\right)\left(x+3\right)}+\dfrac{1}{\left(x+3\right)\left(x+5\right)}+\dfrac{1}{\left(x+5\right)\left(x+7\right)}+\dfrac{1}{\left(x+7\right)\left(x+9\right)}=\dfrac{1}{5}\)\(\Leftrightarrow\dfrac{1}{2}\left(\dfrac{1}{x+1}-\dfrac{1}{x+9}\right)=\dfrac{1}{5}\)
\(\Leftrightarrow\dfrac{8}{\left(x+1\right)\left(x+9\right)}=\dfrac{2}{5}\)
\(\Rightarrow\left(x+1\right)\left(x+9\right)=20\)
\(\Leftrightarrow x^2+10x-11=0\)
\(\Leftrightarrow\left(x-1\right)\left(x+11\right)=0\)
\(\Leftrightarrow x=1\) (nhận) hay \(x=-11\) (nhận).
-Vậy \(S=\left\{1;-11\right\}\)
Câu 21: A
Câu 22: A
Câu 23: C
Câu 24: C
Câu 25: C
