Δ ADE ≈ Δ ABC
DE II BC si AD / DB = AE / EC = k
AD / BC = k /1 ; AD / ( AD + DB ) = k / ( k +1 )
AD / AB = AE / AC = k / ( k +1 )
AE / AC = k / ( k +1 ) ⇒ ( AC - EC ) / AC = k / ( k + 1 )
AC / AC - EC / AC = k / ( k +1 )
1 - k /( k +1 ) = EC / AC
1 / ( k +1 ) = EC / AC