图中所示为一个圆心为 、直径为 的圆。点 和 在圆上。点 使得 和 是圆的切线。

- (a)
已知点 和 分别是 和 的中点,求证:角 。
- (b)
已知三角形 是等边三角形,求证:。
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完整解法
3(a)
1. Identify the angle between the diameter and the tangent.
Since is a diameter and is a tangent to the circle at ,
(tangent radius).
2. Apply the Midpoint Theorem to triangle ABE.
In , is the midpoint of and is the midpoint of .
By the Midpoint Theorem, .
3. Use parallel lines to find angle DCE.
Since and is a transversal,
(corresponding angles).
Therefore, .
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3(b)
1. Identify the angles in the equilateral triangle
Since triangle is equilateral, .
2. Apply the alternate segment theorem to relate the angles
Since is a tangent to the circle at and is a chord, by the alternate segment theorem, the angle between the tangent and the chord is equal to the angle in the alternate segment.
Therefore, .
Since , it follows that .
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