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$$ \angle P = \frac{80^{\circ} - 30^{\circ}}{2} = \frac{50^{\circ}}{2} = \boxed{25^{\circ}} $$

You've finished the course already and are just reviewing, and so this theorem, which is proved in Day 10, isn't something that the kids know about when they're doing the Day 6 lesson. Maybe that's why Prof. Loh tried to use other methods to solve it! ðŸ™‚ That's my guess, but I also know that Prof. Loh likes to use different solution methods to check his work, also. ðŸ™‚

]]>Thus \( \textcolor{purple}{\text{ angle } \frac{a}{2}}\) is half of the measure of \( \textcolor{purple}{\text{ purple arc a,}}\) and \( \textcolor{orange}{\text{ yellow arc b}}\) is half of the measure of \( \textcolor{orange}{\text{ yellow arc b.}}\)

When I read your question, this made me think back to the proof of the Inscribed Angle Theorem which, at the time, only had the special case of an angle with one side as the diameter of the circle. So I've added to that proof by giving an additional visual proof of the Inscribed Angle Theorem for general angles, including angles with a tangent line, like the right-hand \(\textcolor{orange}{\text{ yellow }}\) angle in the diagram.

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M2W2D6-y-part-1-tangent-secant-solution-20-percent.png

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Please check it out! ðŸ™‚

I hope the post with the proof of the Inscribed Angle Theorem will make it more clear why the \(\textcolor{purple}{\text{ purple }}\) angle has measure \( \textcolor{purple}{\frac{a}{2}}\) and both \(\textcolor{orange}{\text{ yellow }}\) angles have measure \(\textcolor{orange}{\frac{b}{2}}\)!

Additionally, this other forum post has a visualization for the formula of an inscribed angle as well as some examples of inscribed angles.

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