Math

A good isosceles triangle problem

Thanks to Dhrubajyoti for the suggestion! A similar problem was asked to class 10 students in India (Q33 from SMO junior 2016). I admit that this one stumped me. Can you figure it out?

Solution presented: post by Thomas Hill on Quora
https://www.quora.com/In-a-triangle-ABC-AB-AC-angle-BAC-100-circ-Let-D-be-a-point-such-that-B-is-an-interior-point-of-AD-and-AD-BC-How-can-you-find-angle-BDC

Other solutions
http://jwilson.coe.uga.edu/EMT668/EMT668.Folders.F97/Rapley/EMT725/100deg/100deg.html
http://jwilson.coe.uga.edu/EMT668/EMAT6680.F99/Wise/essay1/essay1.htm
https://gogeometry.blogspot.com/2011/11/problem-685-isosceles-triangle-100-40.html

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