and on each other's edge. The points and are placed on the circle with center , and the point is placed on the circle with center , such that lies on , and lies on . What is angle , if ?
There are two circles of equal size, with their centers
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Let's call ∠ C D E x . Because △ A B D lies on circle B and it's centerpoint, angles ∠ A D B and ∠ B A D are equal, and therefore ∠ A B D = 1 8 0 − 2 x . Because ∠ A B D + ∠ A B E = 1 8 0 ∘ , ∠ A B E has to be 2 x . Since △ E A B is an iscoceles triangle, because E and B lie on circle A , and A is that circle's centerpoint. Therefore ∠ A E B = 2 x . Also △ C A E is an isosceles triangle, so ∠ C E A = 6 3 ∘ . Now we can determine the value of x using that the sum of the three angles of △ C E D have to add up to 1 8 0 ∘ :
6 3 ∘ + 6 3 ∘ + x + 2 x = 1 8 0 ∘
3 x = 5 4 ∘
x = 1 8 ∘
And therefore ∠ C D E = 1 8 ∘ .