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sin 2B = cot C

Source: Romania NMO 2021 grade 9 P1

April 15, 2023
geometrycircumcircle

Problem Statement

Let ABCABC be an acute-angled triangle with the circumcenter OO. Let DD be the foot of the altitude from AA. If OD∥ABOD\parallel AB, show that sin⁡2B=cot⁡C\sin 2B = \cot C.
Mădălin Mitrofan
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