Câu 13:
ĐKXĐ: \(\begin{cases}x<>0\\ 4-x^2>0\end{cases}\)
=>\(\begin{cases}x<>0\\ x^2<4\end{cases}\Rightarrow\begin{cases}x<>0\\ -2
=>Chọn C
Câu 14:
\(\frac{1}{x^2+x+2}-\frac{1}{x^2+x-2}=1\)
=>\(\frac{x^2+x-2-x^2-x-2}{\left(x^2+x+2\right)\left(x^2+x-2\right)}=1\)
=>\(\left(x^2+x+2\right)\left(x^2+x-2\right)=-4\)
=>\(\left(x^2+x\right)^2-4=-4\)
=>\(\left(x^2+x\right)^2=0\)
=>\(x^2+x=0\)
=>x(x+1)=0
=>x=0 hoặc x=-1
=>Chọn B
Câu 32:
\(\cot\alpha=-a\)
=>\(tan\alpha=-\frac{1}{a}\)
=>\(cos^2\alpha=1:\left(1+\tan^2\alpha\right)=1:\left\lbrack1+\left(-\frac{1}{a}\right)^2\right\rbrack=1:\left\lbrack1+\frac{1}{a^2}\right\rbrack=1:\frac{a^2+1}{a^2}=\frac{a^2}{a^2+1}\)
=>\(cos\alpha=\) \(\frac{-a}{\sqrt{a^2+1}}\)
=>Chọn D
