Butterfly theorem

About the midpoint of a chord of a circle, through which two other chords are drawn From Wikipedia, the free encyclopedia

Butterfly theorem

The butterfly theorem is a classical result in Euclidean geometry, which can be stated as follows:[1]:p. 78

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Butterfly theorem

Let M be the midpoint of a chord PQ of a circle, through which two other chords AB and CD are drawn; AD and BC intersect chord PQ at X and Y correspondingly. Then M is the midpoint of XY.

Proof

Summarize
Perspective
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Proof of Butterfly theorem

A formal proof of the theorem is as follows: Let the perpendiculars XX′ and XX″ be dropped from the point X on the straight lines AM and DM respectively. Similarly, let YY′ and YY″ be dropped from the point Y perpendicular to the straight lines BM and CM respectively.

Since

From the preceding equations and the intersecting chords theorem, it can be seen that

since PM = MQ.

So,

Cross-multiplying in the latter equation,

Cancelling the common term

from both sides of the equation yields

hence MX = MY, since MX, MY, and PM are all positive, real numbers.

Thus, M is the midpoint of XY.

Other proofs exist,[2] including one using projective geometry.[3]

History

Proving the butterfly theorem was posed as a problem by William Wallace in The Gentleman's Mathematical Companion (1803). Three solutions were published in 1804, and in 1805 Sir William Herschel posed the question again in a letter to Wallace. Reverend Thomas Scurr asked the same question again in 1814 in the Gentleman's Diary or Mathematical Repository.[4]


References

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