Răspuns :
[tex]\displaystyle\bf\\\frac{3}{x}=\frac{4}{12}\\\\\textbf{Produsul mezilor este egal cu produsul extremilor.}\\\\4x=3\cdot12\\\\4x=36\\\\x=\frac{36}{4}\\\\\boxed{\bf x=9}[/tex]
[tex]\displaystyle\bf{\dfrac{3}{x} = \dfrac{4}{12}}\\\\\displaystyle{x\cdot4=3\cdot12}\\\\\displaystyle{4x=36}\\\\\displaystyle{x=36:4}\\\\\displaystyle{\boxed{\bf x=9}}\\\\\text{\bf !!! Observatie: produsul mezilor este egal cu produsul extremilor}[/tex]
Răspuns: x = 9