\(\frac{x+4}{20}=\frac{5}{x+4}\Rightarrow\left(x+4\right).\left(x+4\right)=5.20\)
\(\left(x+4\right)^2=100\)
\(\left(x+4\right)^2=10^2\text{ hoặc }\left(x+4\right)^2=\left(-10\right)^2\)
\(\Rightarrow x+4=10\text{ hoặc }x+4=-10\)
\(x=6\text{ hoặc }x=-14\)