Răspuns :
Folosind proprietatea fundamentala a proporțiilor
[tex]\dfrac{5x}{4}=\dfrac{20}{x}\Rightarrow 5x^2=80\Rightarrow x^2=16\Rightarrow x_{1;2}=\pm4[/tex]
[tex]\dfrac{5x}{4}=\dfrac{20}{x}\Rightarrow 5x^2=80\Rightarrow x^2=16\Rightarrow x_{1;2}=\pm4[/tex]