a)
[tex]\it \mathcal{P} _{ABC} = AB + AC +BC = 15+15+BC= 30 +BC \ \ \ (1)
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\mathcal{P} _{ABC} = 50\ m \ \ \ (2)[/tex]
[tex]\it Din\ \ (1), (2) \Longrightarrow 30+BC=50 \Longrightarrow BC=50-30 \Longrightarrow BC = 20 m[/tex]
b)
[tex]\it BE = \dfrac{4}{5}\ din\ BC \Longrightarrow BE = \dfrac{4}{5}\cdot 20 = 4\cdot4 = 16 m
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\mathcal{A}_{BCDE} = BC\cdot BE = 20 \cdot 16 = 320 m^2[/tex]
