2019 AMC 10A Problems/Problem 13
Let be an isosceles triangle with and . Construct the circle with diameter , and let and be the other intersection points of the circle with the sides and , respectively. Let be the intersection of the diagonals of the quadrilateral . What is the degree measure of
Drawing it out, we see and are right angles, as they are inscribed in a semicircle. Using the fact that it is an isosceles triangle, we find . We can find and by the triangle angle sum on and .
Then, we take triangle , and find
~Argonauts16 (Diagram by Brendanb4321)
Solution 2 (Similarity)
Alternatively, we could have used similar triangles. We start similarly to Solution 1.
Drawing it out, we see and are right angles, as they are inscribed in a semicircle. Therefore,
So, by AA Similarity, since and . Thus, we know:
Finally, we know:
Solution 3 (Outside Angles)
Through the property of angles formed by intersecting chords, we find that
Through the Outside Angles Theorem, we find that
Adding the two equations gives us
Since is the diameter, and because is isosceles and , . Thus
Notice that if is degrees, then and must be degrees. Using cyclic quadrilateral properties (or the properties of a subtended arc), we can find that . Thus is, and so is , which is .
angles FDA and FEA are both 90 degrees. angle CAB is so CAB is 70 degrees. Then the answer is -liu4505
Cheap Solution: Create an accurate diagram and measure the angle using a protractor. If you were accurate, the answer is 110 degrees.
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