Find the angle;}

Geometry Level 3

There are two circles of equal size, with their centers A A and B B on each other's edge. The points C C and E E are placed on the circle with center A A , and the point D D is placed on the circle with center B B , such that A A lies on C D |CD| , and B B lies on E D ED . What is angle C D E \angle CDE , if E C D = 6 3 \angle ECD = 63^{\circ} ?

1 7 17^{\circ} 1 9 19^{\circ} 2 0 20^{\circ} 1 8 18^{\circ}

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1 solution

Let's call C D E \angle CDE x x . Because A B D \triangle ABD lies on circle B B and it's centerpoint, angles A D B \angle ADB and B A D \angle BAD are equal, and therefore A B D = 180 2 x \angle ABD = 180 - 2x . Because A B D + A B E = 18 0 \angle ABD + \angle ABE = 180^{\circ} , A B E \angle ABE has to be 2 x 2x . Since E A B \triangle EAB is an iscoceles triangle, because E E and B B lie on circle A A , and A A is that circle's centerpoint. Therefore A E B = 2 x \angle AEB = 2x . Also C A E \triangle CAE is an isosceles triangle, so C E A = 6 3 \angle CEA = 63^{\circ} . Now we can determine the value of x x using that the sum of the three angles of C E D \triangle CED have to add up to 18 0 180^{\circ} :

6 3 + 6 3 + x + 2 x = 18 0 63^{\circ} + 63^{\circ} + x + 2x = 180^{\circ}

3 x = 5 4 3x = 54^{\circ}

x = 1 8 x = 18^{\circ}

And therefore C D E = 1 8 \angle CDE = 18^{\circ} .

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