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2017 AMC 10B Problem 22

Problem 22 of 25HarderGeometry

The diameter AB\overline{AB} of a circle of radius 22 is extended to a point DD outside the circle so that BD=3.BD=3. Point EE is chosen so that ED=5ED=5 and line EDED is perpendicular to line AD.AD. Segment AE\overline{AE} intersects the circle at a point CC between AA and E.E. What is the area of ABC?\triangle ABC?

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Solution

Since the radius is 22 and BD=3,BD =3, we have AD=7.AD = 7. Since ED=5ED = 5 and the angle at DD is a right angle, the area of ADEADE is 572=352.\dfrac{5\cdot 7}{2} = \dfrac{35}{2} . By the Pythagorean Theorem, AE=52+72=74.AE=\sqrt{5^2+7^2}=\sqrt{74}. Also, ACB\angle ACB is a right angle because ABAB is a diameter. The triangles share the angle at A,A, so ABCAED\triangle ABC\sim\triangle AED by angle-angle similarity. Their corresponding hypotenuses are AB=4AB=4 and AE=74,AE=\sqrt{74}, so their area ratio is [ABC][AED]=(474)2=837.\dfrac{[ABC]}{[AED]}=\left(\dfrac4{\sqrt{74}}\right)^2=\dfrac8{37}. Therefore, [ABC]=837352=14037.[ABC]=\dfrac8{37}\cdot\dfrac{35}{2}=\dfrac{140}{37}. Thus, the correct answer is D .

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Concepts: inscribed angle · similarity · area ratio

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.