Can you find the angle value?

Geometry Level 3

What is the value (in degrees) of the red angle ?

Note : Answer -1 if it can't be determined, for instance if it depends on the length A C AC .


The answer is 15.

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2 solutions

Jeremy Galvagni
Aug 20, 2018

C D E \triangle CDE is equilateral so the angle at D D is 6 0 60^{\circ} . This is a central angle at D D but an inscribed angle at C C , so C = 3 0 \angle C=30^{\circ} .

Do it again on the other circle where now C C is a central angle but A A is an inscribed angle. A = 1 5 \angle A = \boxed{15^{\circ}}

You mean C = 30 ° \angle C = 30° ...

Antoine G - 2 years, 9 months ago

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Of course. Thank you.

Jeremy Galvagni - 2 years, 9 months ago

Since C D E \triangle CDE is equilateral, D C E = 6 0 \angle DCE=60^{\circ} . And as E F \overline{EF} is diameter, E C F = 9 0 \angle ECF=90^{\circ} . D C F = 3 0 C A F = 1 5 \Rightarrow \angle DCF=30^{\circ} \Rightarrow \angle CAF=15^{\circ}

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