What is the point of E?

Geometry Level 2

Circles Γ 1 \Gamma_1 and Γ 2 \Gamma_2 intersect at 2 distinct points A A and B B . A line l l through A A intersects Γ 1 \Gamma_1 and Γ 2 \Gamma_2 at points C C and D , D, respectively, such that C C is not in Γ 2 \Gamma_2 and D D is not in Γ 1 \Gamma_1 . Point E E is the intersection of the tangent to Γ 1 \Gamma_1 at C C and the tangent to Γ 2 \Gamma_2 at D D .

If C B D = 7 1 , \angle CBD = 71^\circ, what is the measure (in degrees) of C E D ? \angle CED?


The answer is 109.

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2 solutions

Calvin Lin Staff
May 13, 2014

From the condition that C C is not in Γ 2 \Gamma_2 and D D is not in Γ 1 \Gamma_1 , we get that E E is on the opposite side of C D CD as compared to B B . From the alternate segment theorem, D C E = A C E = A B C \angle DCE = \angle ACE = \angle ABC , and C D E = A D E = A B D \angle CDE = \angle ADE = \angle ABD . Thus C E D = 18 0 C D E D C E = 18 0 A B D A B C = 18 0 C B D = 18 0 7 1 = 10 9 \begin{aligned} \angle CED &= 180^\circ - \angle CDE - \angle DCE \\ &= 180^\circ - \angle ABD-\angle ABC \\ &= 180^\circ - \angle CBD \\ &= 180^\circ - 71 ^\circ \\ &= 109 ^\circ \\ \end{aligned}

Jed Aguirre
May 20, 2014

Quadrilateral BCED will be formed from these tangent lines such that angle BCE and angle BDE are 90 degrees each (definition of tangent lines in a circle). Also, the sum of the measure of interior angles of a quadrilateral is 360, which includes angles BCE, BDE, CBD and CED. Since BCE = 90, BDE = 90, and CBD = 71, then m<CED = 360-90-90-71=109 degrees.

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