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Home » Circles » Chords » Common Chord of Two Circles

Common Chord of Two Circles

Last updated: Sep 30, 2019 by Ido Sarig · This website generates income via ads and uses cookies · Terms of use · Privacy policy

In today's lesson, we will show that a line connecting the centers of two intersecting circles is a perpendicular bisector of the common chord of the two circles, connecting the intersection points.

When two circles intersect, we can connect the two intersection points and create a common chord.

If we connect the centers of these two circles, the connecting line will be a perpendicular bisector of the common chord, as we will now show.

Problem

Circles O and Q intersect at points A and B. Show that the line OQ connecting the centers of the two circles bisects AB and is perpendicular to AB.

common chord

Strategy

Many problems involving circles and chords make use of the fact that all of a circle's radii are equal, and use this fact to show congruent triangles, so let draw those radii:

common chord + kite

This immediately suggests we could solve this geometry problem by first showing that triangles △AOQ and △BOQ are congruent (using the Side-Side-side postulate - We've actually already done that as an exercise here - Congruent Triangles in a Circle ).

And after doing that, we could easily show that triangles △AOC and △BOC are congruent using the Side-Angle-Side postulate, having just shown that ∠AOC is congruent to ∠BOC as corresponding angles in the congruent triangles △AOQ and △BOQ).

But there's an even easier way. The radii of each circle are equal, OA=OB and QA=QB, which means that OAQB is a kite (just like it looks in the diagram!). And we know that in a kite, the diagonal connecting the corners between the equal sides bisects the other diagonal. We also know that the diagonals are perpendicular to each other. So we are done!

Proof

(1) OA=OB //radii of a circle are equal
(2) QA=QB //radii of a circle are equal
(3) OAQB is a kite //(1) , (2) , definition of a kite
(4) AC=CB // (3), diagonal connecting the angles formed by the equal sides bisects the other diagonal
(5) AB⊥OQ // (3), diagonals of a kite are perpendicular

« Distance Between the Centers of Overlapping Congruent Circles
The Diagonals of a Kite are Perpendicular to Each Other »

About the Author

Ido Sarig is a high-tech executive with a BSc degree in Computer Engineering. His goal is to help you develop a better way to approach and solve geometry problems. You can contact him at [email protected]

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About

Welcome to Geometry Help! I'm Ido Sarig, a high-tech executive with a BSc degree in Computer Engineering and an MBA degree in Management of Technology. I'm here to tell you that geometry doesn't have to be so hard! My goal with this website is to help you develop a better way to approach and solve geometry problems, even if spatial awareness is not your strongest quality. Read More…

Geometry Topics

  • Area of Geometric Shapes
  • Circles
    • Arcs, Angles, and Sectors
    • Chords
    • Inscribed Shapes
    • Tangent Lines
  • Lines and Angles
    • Intersecting Lines and Angles
    • Parallel Lines
    • Perpendicular lines
  • Pentagons and Hexagons
  • Perimeter of Geometric Shapes
  • Polygons
  • Quadrangles
    • Kites (Deltoids)
    • Parallelograms
    • Rectangles
    • Rhombus
    • Squares
    • Trapezoids
  • Triangles
    • Congruent Triangles
    • Equilateral Triangles
    • Isosceles Triangles
    • Pythagorean Theorem
    • Right Triangles
    • Similar Triangles
    • Triangle Inequalities

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