Geometry: Langley's Problem, Interactive Illustration

Elements: Isosceles Triangle, 20, 50, 60, 80 Degrees, Angle.

In an isosceles triangle ABC (AB = BC), E is on AB and D is on BC. If angle B = 20 degrees, angle DAC = 60 degrees, and angle ACE = 50 degrees, find the measure of angle ADE. Click the next (play) button below to start. See also: HTML5 version

The famous Langley problem is a type of problem called "Adventitious Angles" problem because it is a matter of chance that we will be able to solve this problem (drawing auxiliary lines). It appears to be an easy problem, but it is deceivingly difficult.

• Reference: The Mathematical Gazette, UK
• Author: Edward M. Langley
• Title: A problem
• Page: 173
• Issue: October 1922
• Category: M Note 644

See also:
Geometry
Dynamic Geometry
Adobe Flash
Isosceles Triangle 80-20-80
Artwork of Langley's problem
Jigsaw Puzzle: Langley's Problem: 40 Quadrilateral Pieces
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