MN, NP si MP sunt linii mijlocii in ABC
[tex]MN=\frac{BC}{2} \\ \\ NP=\frac{AB}{2} \\ \\ MP=\frac{AC}{2} \\ \\ \mathcal{P}_{\triangle MNP}= \\ \\ MN+NP+MP= \\ \\ \frac{BC}{2}+\frac{AB}{2}+\frac{AC}{2}= \\ \\ \frac{BC+AB+AC}{2}= \\ \\ \frac{\mathcal{P}_{\triangle ABC}}{2}= \\ \\ \frac{20dm}{2}=10~dm[/tex]