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 .
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You mean ∠ C = 3 0 ° ...
Since △ C D E is equilateral, ∠ D C E = 6 0 ∘ . And as E F is diameter, ∠ E C F = 9 0 ∘ . ⇒ ∠ D C F = 3 0 ∘ ⇒ ∠ C A F = 1 5 ∘
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△ C D E is equilateral so the angle at D is 6 0 ∘ . This is a central angle at D but an inscribed angle at C , so ∠ C = 3 0 ∘ .
Do it again on the other circle where now C is a central angle but A is an inscribed angle. ∠ A = 1 5 ∘