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
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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

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Isosceles Triangle 80-20-80

Artwork of Langley's problem

Jigsaw Puzzle: Langley's Problem: 40 Quadrilateral Pieces

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40 Quadrilateral Pieces.

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