MathDB

9.3

Part of 2009 All-Russian Olympiad Regional Round

Problems(1)

right angle wanted, 3 altitudes

Source: All-Russian MO 2009 Regional 9.3

8/27/2024
In an acute triangle ABCABC the altitudes AA1AA_1, BB1BB_1, CC1CC_1 are drawn. A line perpendicular to side ACAC and passing through a point A1A_1, intersects the line B1C1B_1C_1 at point DD. Prove that angle ADCADC is right.
geometryright angleperpendicular