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Bunuel
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Solution


Given:
    • In the given figure, BD is perpendicular to AC
    • AC = 21

To find:
    • The length of AD

Approach and Working:
As BD is perpendicular to AC, we can say triangle DBC and triangle DBA are both right-angled triangles.

In triangle DBC,
    • DC = 13 and BC = 5
    • Therefore, from Pythagoras Theorem, we can say DB = \(\sqrt{13^2 – 5^2}\) = 12

In triangle DBA,
    • DB = 12 and BA = 21 – 5 = 16
    • Therefore, from Pythagoras Theorem, we can say AD = \(\sqrt{12^2 + 16^2}\) = 20

Hence, the correct answer is option C.

Answer: C

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Bunuel

In the figure above, if BD is perpendicular to AC and AC = 21, then AD =

A. 12
B. 13
C. 20
D. 21
E. 25

Attachment:
2019-02-25_1345.png

Use pythagorean triplets:

Triangle BCD is a right triangle. Since hypotenuse is 13 and one leg is 5, the other leg BD = 12 (since 5-12-13 is a pythagorean triplet)

Triangle ABD is also a right triangle such that AB = 21 - 5 = 16 and BD = 12. The two legs are 12 and 16 (in the ratio 3:4). So this is a multiple of pythagorean triplet 3-4-5 with a multiplier of 4. The hypotenuse AD will be 5*4 = 20

Answer (C)
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