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B N A M C
[tex]a)...\,AC=5cm, \frac{AB}{BC}=\frac{2}{3}\rightarrow\,\,\frac{AB}{AB+5}=\frac{2}{3}\,,\\de\,unde,\,3AB=2AB+10\,,\,\,\,[AB]=10cm\,,\\
b)...\,[BC]=10cm+5cm=15cm\,\,,\\
c)...\,AM=AC/2=2,5cm\,\,\,\,,\,\,\,BN=BC/2=7,5cm\,,\\
.\,\,\,\,\,\,\,\,\,\,\,\,[MN]=BC-BN-MC=10-5-2,5=2,5(cm)\,.[/tex]