Are you too tired to chase angles?

Geometry Level 3

Let ABC be an isosceles triangle A B = A C AB=AC with B A C = 2 0 \angle BAC = 20^{\circ} . Point D D is on side A C AC such that D B C = 6 0 \angle DBC = 60^{\circ} . Point E E is on side A B AB such that E C B = 5 0 \angle ECB = 50^{\circ} . Find the measure of E D B \angle EDB in degrees.


The answer is 30.

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1 solution

Alan Yan
Oct 24, 2015

Construct a line parallel to B C BC through D D and call its intersection with A B AB , F F . Connect F C FC and call its intersection with B D BD , G G . Since A B C \triangle ABC is isosceles and D B C = 6 0 \angle DBC = 60^{\circ} , we know that B G C \triangle BGC and F D G \triangle FDG are equilateral triangles.Also notice that B E C = 5 0 \angle BEC = 50^{\circ} which implies B E C \triangle BEC is B-isosceles. Since B G C \triangle BGC is equilateral, this implies that B E = B C = B G BE = BC = BG which implies that B E G \triangle BEG is isosceles which implies that B G E = 8 0 \angle BGE = 80^{\circ} . Due to supplementary angles, we find that F G E = 4 0 \angle FGE = 40^{\circ} . Also notice that B F C = 4 0 \angle BFC = 40^{\circ} which implies that E F G \triangle EFG is isosceles which implies that F E G D FEGD is a kite which gives us an answer of 3 0 \boxed{30^{\circ}} since the diagonals are perpendicular.

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