Four Points in The Plane

We are given points A,B,C and D in the plane such that AD=13, whereas AB=BC=AC=CD=10. Compute the measure of \angle{ADB} in degrees.
Source: NCTM Mathematics Teacher


C is the center of a circle of radius 10 passing through points A,B,D. The central \angle{BCA}=60^\circ because \triangle{ABC} is equilateral. \angle{ADB} is an inscribed angle that intercepts arc AB,

Answer: 30^\circ


About mvtrinh

Retired high school math teacher.
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