Respuesta :

Answer : The demonstration is below :)

Step-by-step explanation :

We use Pythagoras' theorem :

  • In the triangle ABC we have :

AB² = AC² - BC² = 15² - 9² = 144 = 12²

  • In the triangle ABD we have :

DB² = AD² - AB² = 13² - 12² = 5²

cos(a) = BD/AD = 5/13

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