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Difference between revisions of "Steiner's Theorem"

(Got kicked out of computer lab because of halo players; came back to finish the proof)
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Let <math>E</math> be the intersection of <math>AD</math> and <math>BC</math>, <math>F</math> be the midpiont of <math>AB</math>, <math>G</math> be the midpoint of <math>CD</math>, and <math>H</math> be the intersection of <math>AC</math> and <math>BD</math>. We now claim that <math>\triangle EAF \sim \triangle EDG</math>. First note that, since <math>\angle DEC=\angle AEB</math> and <math>\angle EAB=\angle EDC</math> [this is because <math>AB\parallel CD</math>], we have that <math>\triangle EDC\sim \triangle EAB</math>. Then <math>\frac{EA}{AB}=\frac{ED}{DC}</math>, and <math>\frac{AF}{AB}=\frac{DG}{DC}=\frac{1}{2}</math>, so <math>\frac{EA}{AF}=\frac{ED}{DG}</math>. We earlier stated that <math>\angle EDC=\angle EAB</math>, so we have that <math>\triangle EAF \sim \triangle EDG</math> from SAS similarity. We have that <math>E</math>, <math>A</math>, and <math>D</math> are collinear, and since <math>F</math> and <math>G</math> are on the same side of line <math>AD</math>, we can see that <math>\triangle EAF \sim \triangle EDG</math> from SAS. Therefore <math>EF \parallel EG</math>, so <math>E</math>, <math>F</math>, and <math>G</math> are collinear.
 
Let <math>E</math> be the intersection of <math>AD</math> and <math>BC</math>, <math>F</math> be the midpiont of <math>AB</math>, <math>G</math> be the midpoint of <math>CD</math>, and <math>H</math> be the intersection of <math>AC</math> and <math>BD</math>. We now claim that <math>\triangle EAF \sim \triangle EDG</math>. First note that, since <math>\angle DEC=\angle AEB</math> and <math>\angle EAB=\angle EDC</math> [this is because <math>AB\parallel CD</math>], we have that <math>\triangle EDC\sim \triangle EAB</math>. Then <math>\frac{EA}{AB}=\frac{ED}{DC}</math>, and <math>\frac{AF}{AB}=\frac{DG}{DC}=\frac{1}{2}</math>, so <math>\frac{EA}{AF}=\frac{ED}{DG}</math>. We earlier stated that <math>\angle EDC=\angle EAB</math>, so we have that <math>\triangle EAF \sim \triangle EDG</math> from SAS similarity. We have that <math>E</math>, <math>A</math>, and <math>D</math> are collinear, and since <math>F</math> and <math>G</math> are on the same side of line <math>AD</math>, we can see that <math>\triangle EAF \sim \triangle EDG</math> from SAS. Therefore <math>EF \parallel EG</math>, so <math>E</math>, <math>F</math>, and <math>G</math> are collinear.
  
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Now consider triangles <math>HBA</math> and <math>HDC</math>. Segments <math>AC</math> and <math>BD</math> are transversal lines, so it's not hard to see that <math>\triangle HAB \sim \triangle HCD</math>. It's also not hard to show that <math>\triangle HBE</math> \sim \triangle HDF<math> by SAS similarity. Therefore </math>\angle BHE=\angle DHF<math>, which implies that </math>F<math>, </math>G<math>, and </math>H$ are collinear. This completes the proof.
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Now consider triangles <math>HBA</math> and <math>HDC</math>. Segments <math>AC</math> and <math>BD</math> are transversal lines, so it's not hard to see that <math>\triangle HAB \sim \triangle HCD</math>. It's also not hard to show that <math>\triangle HBE \sim \triangle HDF</math> by SAS similarity. Therefore <math>\angle BHE=\angle DHF</math>, which implies that <math>F</math>, <math>G</math>, and <math>H</math> are collinear. This completes the proof.
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==See also==
 
==See also==

Revision as of 07:03, 5 July 2011

Steiner's Theorem states that in a trapezoid $ABCD$ with $AB\parallel CD$ and $AD\nparallel BC$, we have that the midpoint of $AB$ and $CD$, the intersection of diagonals $AC$ and $BD$, and the intersection of the sides $AD$ and $BC$ are collinear.

Proof

Let $E$ be the intersection of $AD$ and $BC$, $F$ be the midpiont of $AB$, $G$ be the midpoint of $CD$, and $H$ be the intersection of $AC$ and $BD$. We now claim that $\triangle EAF \sim \triangle EDG$. First note that, since $\angle DEC=\angle AEB$ and $\angle EAB=\angle EDC$ [this is because $AB\parallel CD$], we have that $\triangle EDC\sim \triangle EAB$. Then $\frac{EA}{AB}=\frac{ED}{DC}$, and $\frac{AF}{AB}=\frac{DG}{DC}=\frac{1}{2}$, so $\frac{EA}{AF}=\frac{ED}{DG}$. We earlier stated that $\angle EDC=\angle EAB$, so we have that $\triangle EAF \sim \triangle EDG$ from SAS similarity. We have that $E$, $A$, and $D$ are collinear, and since $F$ and $G$ are on the same side of line $AD$, we can see that $\triangle EAF \sim \triangle EDG$ from SAS. Therefore $EF \parallel EG$, so $E$, $F$, and $G$ are collinear.

Now consider triangles $HBA$ and $HDC$. Segments $AC$ and $BD$ are transversal lines, so it's not hard to see that $\triangle HAB \sim \triangle HCD$. It's also not hard to show that $\triangle HBE \sim \triangle HDF$ by SAS similarity. Therefore $\angle BHE=\angle DHF$, which implies that $F$, $G$, and $H$ are collinear. This completes the proof.

See also

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