In triangle ABC, angle C = 15 degrees, point D is marked on the side AC, so that angle ABD = 12 degrees and angle ADB = 80 degrees. Prove that triangle ABC is not right-angled.
The ADB angle is adjacent to the CDB angle, then the ADB angle = (180 – 80) = 100.
The angle BAD of triangle ABD is: BAD = (180 – 100 – 12) = 68.
Then the angle ABC = (180 – 68 – 15) = 97.
Triangle ABC is not rectangular, as required.
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