a: \(\Leftrightarrow x^2+1=4\)
hay \(x\in\left\{\sqrt{3};-\sqrt{3}\right\}\)
b: \(\Leftrightarrow x^2+5x+20=16\)
\(\Leftrightarrow x^2+5x+4=0\)
=>(x+1)(x+4)=0
=>x=-1 hoặc x=-4
\(< =>x^2+1=4< =>x^2=3< =>x=\pm\sqrt{3}\)
\(< =>x^2+5x+20=16< =>x^2+5x+\text{4}=0< =>x^2+4x+x+4=0< =>\left(x+1\right)\left(x+4\right)=0< =>\left[{}\begin{matrix}x+1=0< =>x=-1\\x+4=0< =>x=-4\end{matrix}\right.\)
\(\left(\sqrt{x^2+1}\right)^2=2^2\\x^2+1=4\\ x^2=3 \)
=> \(x=\sqrt{3};x=-\sqrt{3}\)
\(\left(\sqrt{x^2+5x+20}\right)^2=4^2\\ x^2+5x+20=16\\ x^2+5x+4=0\)
=> x=-1; x =-4
