Midpoint theorem (triangle): Difference between revisions

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{{Short description|Geometric theorem involving midpoints on a triangle}}

[[File:Midpoint theorem.svg|thumb|upright=1.25|<math> \begin{align} &\text{D and E midpotsmidpoints of AC and BC}\\ \Rightarrow \, &DE \parallel AB\text{ and } 2|DE|=|AB|\end{align}</math>]]

The '''midpoint theorem''' or '''midline theorem''' states that if you connect the midpoints of two sides of a triangle then the resulting line segment will be parallel to the third side and have half of its length. The midpoint theorem generalizes to the [[intercept theorem]], where you rather than using midpoints partition both sides in the same ratio.

The '''midpoint theorem''', '''midsegment theorem''', or '''midline theorem''' states that if the midpoints of two sides of a triangle are connected, then the resulting line segment will be parallel to the third side and have half of its length. The midpoint theorem generalizes to the [[intercept theorem]], where rather than using midpoints, both sides are partitioned in the same ratio.<ref>{{Cite book |last=Clapham |first=Christopher |title=The concise Oxford dictionary of mathematics: clear definitions of even the most complex mathematical terms and concepts |last2=Nicholson |first2=James |date=2009 |publisher=Oxford Univ. Press |isbn=978-0-19-923594-0 |edition=4th |series=Oxford paperback reference |___location=Oxford |pages=297}}</ref><ref>{{Cite book |last=French |first=Doug |title=Teaching and learning geometry: issues and methods in mathematical education |date=2004 |publisher=Continuum |isbn=978-0-8264-7362-2 |___location=London; New York |pages=81–84 |oclc=ocm56658329}}</ref>

The converse of the theorem is true as well. That is if you draw a line is drawn through the midpoint of triangle side parallel to another triangle side then the line will bisect the third side of the triangle.

The triangle formed by the three parallel lines through the three midpoints of sides of a triangle is called its [[medial triangle]].

==Proof==

===Proof by construction===

{{Math proof|proof=[[File:Midpoint Theorem proof.png|thumb|304x304px]]

'''Given''': In a <math>\triangle ABC </math> the points M and N are the midpoints of the sides AB and AC respectively.

'''[[Geometric Construction|Construction]]''': MN is extended to D where MN=DN, join C to D.

'''To Prove''':

*<math>MN\parallel BC</math>

*<math>MN={1\over 2}BC</math>

'''Proof''':

*<math>AN=CN</math> (given)

*<math>\angle ANM=\angle CND</math> (vertically opposite angle)

*<math>MN=DN</math> (constructible)

Hence by [[Side angle side]].

:<math>\triangle AMN\cong\triangle CDN </math>

Therefore, the corresponding sides and angles of congruent triangles are equal

*<math>AM=BM=CD</math>

*<math>\angle MAN=\angle DCN</math>

[[Transversal (geometry)|Transversal]] AC intersects the lines AB and CD and alternate angles ∠MAN and ∠DCN are equal. Therefore

*<math>AM\parallel CD\parallel BM</math>

Hence BCDM is a [[parallelogram]]. BC and DM are also equal and parallel.

*<math>MN\parallel BC</math>

*<math>MN={1\over 2}MD={1\over 2}BC</math>,

[[Q.E.D.]]

}}

===Proof by similar triangles===

{{Math proof|proof=[[File:Midpoint theorem.svg|thumb|304x304px]]

Let D and E be the midpoints of AC and BC.

'''To prove:'''

* <math>DE\parallel AB</math>,

* <math>DE = \frac{1}{2}AB</math>.

'''Proof:'''

<math>\angle C</math> is the common angle of <math>\triangle ABC</math> and <math>\triangle DEC</math>.

Since DE connects the midpoints of AC and BC, <math>AD=DC</math>, <math>BE=EC</math> and <math>\frac{AC}{DC}=\frac{BC}{EC}=2.</math> As such, <math>\triangle ABC</math> and <math>\triangle DEC</math> are [[Similarity (geometry)|similar]] by the SAS criterion.

Therefore, <math>\frac{AB}{DE}=\frac{AC}{DC}=\frac{BC}{EC}=2,</math> which means that <math>DE=\frac{1}{2}AB.</math>

Since <math>\triangle ABC</math> and <math>\triangle DEC</math> are similar and <math>\triangle DEC \in \triangle ABC</math>, <math>\angle CDE = \angle CAB</math>, which means that <math>AB\parallel DE</math>.

[[Q.E.D.]]

}}

==See also==

* [[Median of the trapezoid theorem]]

==References==

{{reflist}}

*{{cite book |title=Teaching and Learning Geometry|first1=Doug|last1=French|author1-link= Doug French|publisher=Bloomsbury|year=2004|isbn=9780826473622|pages= 81–84}} ({{Google books|j1nUAwAAQBAJ|online copy|page=81}})

*{{cite book |title=The Concise Oxford Dictionary of Mathematics|first1=Christopher|last1=Clapham|first2=James|last2=Nicholson|author1-link=Christopher Clapham|publisher=Oxford University Press|year=2009|isbn=9780199235940|pages=297}} ({{Google books|ZsiSvE0Z3s4C|online copy|page=297}})

==External links ==

*[http://math.fau.edu/yiu/Oldwebsites/MSTHM2014/Supplement0627.pdf ''The midpoint theorem and its converse'']

*[https://www.edumple.com/cbse-class-9/mathmatics/motivate-in-a-parallelogram-the-diagonals-bisect-each-other-and-conversely/notes/saira_2419 The Mid-Point Theorem'']

*[https://www.youtube.com/watch?v=Q457DRC33zY ''Midpoint theorem and converse Euclidean explained Grade 10+12 ''] (video, 5:28 mins)

*[https://proofwiki.org/wiki/Midline_Theorem ''midpoint theorem''] at the Proof Wiki

[[Category:Theorems about triangles]]