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TOT 313 1991 Autumn O S3 <C obtuse if AD/DC = AB/ BC

Source:

June 9, 2024
geometryobtuseanglesratioequal ratiosequal ratio

Problem Statement

Point DD lies on side ABAB of triangle ABCABC, and ADDC=ABBC.\frac{AD}{DC} = \frac{AB}{BC}. Prove that angle CC is obtuse.
(Sergey Berlov)
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