Do intersecting circles always have equal angles at the circumference?

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Homework Help Overview

The discussion revolves around the geometric properties of intersecting circles, specifically focusing on the angles subtended by a common chord at the circumference. The original poster questions whether these angles are always equal, regardless of the circles' radii, and seeks clarification on any special cases that might apply.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationship between angles subtended by a common chord in intersecting circles, questioning if these angles remain equal under various conditions. There is mention of moving points along the circles and the implications for angle equality. Some participants suggest examining specific configurations, such as when the centers of the circles form a rhombus.

Discussion Status

The discussion is active, with participants engaging in reasoning about the properties of angles in intersecting circles. Some have proposed proofs and counterexamples, while others are questioning assumptions and seeking clarification on specific cases. There is no explicit consensus yet, but several lines of inquiry are being explored.

Contextual Notes

Participants are considering various configurations of the circles and the implications of their centers' positions. There is an acknowledgment of potential missing links in reasoning that could affect the conclusions drawn about angle equality.

mattg443
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I was wondering if the common chord of two intersecting circles subtends an equal angle in both circles at the circumference (in no special cases i.e different radii circles etc)

If not, are there any special case(s) where this would work, making these two triangles similar or ABDC is a kite/special quadrilateral...

(see diagram please)

Thanks!
 

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If you move D along its circle, the angle doesn't change.
 
Yes, but is angle ADC equal to the angle subtended by the same chord in the other circle (angle ABC)
 
robphy said:
If you move D along its circle, the angle doesn't change.

mattg443 said:
Yes, but is angle ADC equal to the angle subtended by the same chord in the other circle (angle ABC)

Did you try looking at the picture if you move D like robphy suggested??
 
when D is in the circle, ABC=ADC

I have come up with a proof which says otherwise to robphy for when B and D are on different circles, UNLESS is it is missing a link (which is likely)

I added a centre to each of the circles and found that angle ADC is only equal to angle ABC is AO1CO2 (where O1 & 02 are the centres of the circles) is a rhombus (opposite angles equal) so that 2α=2β

Let my know the missing link I am making then for it to be true for all cases, as suggested by robphy.
 

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