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Japan Mathematical Olympiad

Japan geometry

Problem

Let be an acute triangle, be the foot of the perpendicular from to , and be the midpoint of . Let be a point on the segment such that . Given and , find the value of .
Solution
78°

Triangles and are similar since , thus . Moreover, we have , since and is the midpoint of . Then triangles and are similar since , thus . By , we obtain Since and the sum of the three angles of is equal to , we obtain . Thus the answer is .
Final answer
78°

Techniques

TrianglesTriangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleAngle chasing