Stewart's Theorem: Difference between revisions

Latest revision as of 06:41, 19 September 2025

Statement

Given any triangle $\triangle ABC$ with sides of length $a, b, c$ and opposite vertices $A$ , $B$ , $C$ , respectively, then if cevian $AD$ is drawn so that $BD = m$ , $DC = n$ and $AD = d$ , we have that $b^2m + c^2n = amn + d^2a$ . (This is also often written $man + dad = bmb + cnc$ , a phrase which invites mnemonic memorization, i.e. "A man and his dad put a bomb in the sink.") That is Stewart's Theorem.

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Proof

Proof 1

Applying the Law of Cosines in triangle $\triangle ABD$ at angle $\angle ADB$ and in triangle $\triangle ACD$ at angle $\angle CDA$ , we get the following equations:

$n^{2} + d^{2} - 2nd\cos{\angle CDA} = b^{2}$
$m^{2} + d^{2} - 2md\cos{\angle ADB} = c^{2}$

Because angles $\angle ADB$ and $\angle CDA$ are supplementary, $m\angle ADB = 180^\circ - m\angle CDA$ . We can therefore solve both equations for the cosine term. Using the trigonometric identity $\cos{\theta} = -\cos{(180^\circ - \theta)}$ gives us

$\frac{n^2 + d^2 - b^2}{2nd} = \cos{\angle CDA}$
$\frac{c^2 - m^2 -d^2}{2md} = \cos{\angle CDA}$

Setting the two left-hand sides equal and clearing denominators, we arrive at the equation: $c^{2}n + b^{2}m=m^{2}n +n^{2}m + d^{2}m + d^{2}n$ . However, $m+n = a$ so $m^2n + n^2m = (m + n)mn = amn$ and $d^2m + d^2n = d^2(m + n) = d^2a.$ This simplifies our equation to yield $man + dad = bmb + cnc,$ as desired.

Proof 2 (Pythagorean Theorem)

Let the altitude from $A$ to $BC$ meet $BC$ at $H$ . Let $AH=h$ , $CH=x$ , and $HD=y$ .

We can apply the Pythagorean Theorem on $\triangle AHC$ and $\triangle AHD$ to yield $h^2 = b^2 - x^2 = d^2 - y^2$ and then solve for $b$ to get $b^2 = d^2 + x^2 - y^2$ .

Doing the same for $\triangle AHB$ and $\triangle AHD$ gives us:

$h^2 = c^2 - (m + y)^2 = d^2 - y^2,$ then we solve for $c$ to get $c^2 = d^2 + m^2 + 2my$ .

Now multiple the first expression by $m$ and the second by $n$ :

\begin{align*} mb^2 &= md^2 + m(x^2 - y^2)\\ nc^2 &= nd^2 + m^2n + 2mny \end{align*}

Next, we add these two expressions:

$mb^2 + nc^2 = md^2 + m(x^2 - y^2) + nd^2 + m^2n + 2mny.$

Then simplify as follows (we reapply $x + y = n$ a few times while factoring):

\begin{align*} mb^2 + nc^2 &= (m + n)d^2 + m(x+y)(x - y) + mn(n + 2y)\\ &= (m + n)d^2 + mn(x - y) + mn(n + 2y)\\ &= (m + n)d^2 + mn(x + y + n)\\ &= (m + n)d^2 + mn(m + n)\\ &= (m + n)(d^2 + mn). \end{align*}

Rearranging the equation gives Stewart's Theorem: $man+dad = bmb+cnc$ .

Proof 3 (Barycentrics)

Let the following points have the following coordinates:

$A: (1,0,0)$

$B: (0,1,0)$

$C: (0,0,1)$

$D: \left(0, \frac{n}{m+n},\frac{m}{m+n}\right)$

Our displacement vector $\overrightarrow{AD}$ has coordinates $\left(1, -\frac{n}{m+n}, -\frac{m}{m+n}\right)$ . Plugging this into the barycentric distance formula, we obtain $d^2=-(m+n)^2 \left(\frac{mn}{(m+n)^2} \right)-b^2 \left ( -\frac{m}{m+n} \right)-c^2 \left(-\frac{n}{m+n}\right)=-mn+\frac{b^2m+c^2n}{m+n}$ Multiplying by $m+n$ , we get $d^2(m+n)+mn(m+n)=b^2m+c^2n$ . Substituting $m+n$ with $a$ , we find Stewart's Theorem: $\boxed{d^2a+amn=b^2m+c^2n}$

Video Proof

TheBeautyofMath

@@ Line 1: / Line 1: @@
 == Statement ==
-''(awaiting image)''<br>
-If a [[cevian]] of length t is drawn and divides side a into segments m and n, then
+Given any [[triangle]] <math>\triangle ABC</math> with sides of length <math>a, b, c</math> and opposite [[vertex|vertices]] <math>A</math>, <math>B</math>, <math>C</math>, respectively, then if [[cevian]] <math>AD</math> is drawn so that <math>BD = m</math>, <math>DC = n</math> and <math>AD = d</math>, we have that <math>b^2m + c^2n = amn + d^2a</math>.  (This is also often written <math>man + dad = bmb + cnc</math>, a phrase which invites mnemonic memorization, i.e. "A man and his dad put a bomb in the sink.") That is Stewart's Theorem.
-<br><center><math>cmc+bmb=man+cnc</math></center><br>
+[[Image:Stewart's_theorem.png]]
 == Proof ==
-For this proof we will use the law of cosines and the identity <math>\cos{\theta} = -\cos{180 - \theta}</math>.
-Label the triangle <math>ABC</math> with a cevian extending from <math>A</math> onto <math>BC</math>, label that point <math>D</math>. Let CA = n Let DB = m. Let AD = t. We can write two equations:
+=== Proof 1 ===
-*<math> n^{2} + t^{2} - nt\cos{\angle CDA} = b^{2} </math>
-*<math> m^{2} + t^{2} + mt\cos{\angle CDA} = c^{2} </math>
+Applying the [[Law of Cosines]] in triangle <math>\triangle ABD</math> at [[angle]] <math>\angle ADB</math> and in triangle <math>\triangle ACD</math> at angle <math>\angle CDA</math>, we get the following equations:
-When we write everything in terms of cos(CDA) we have:
+* <math>n^{2} + d^{2} - 2nd\cos{\angle CDA} = b^{2}</math>
-*<math> \frac{n^2 + t^2 - b^2}{nt} = \cos{\angle CDA}</math>
+* <math>m^{2} + d^{2} - 2md\cos{\angle ADB} = c^{2}</math>
-*<math> \frac{c^2 - m^2 -t^2}{mt} = \cos{\angle CDA}</math>
+Because angles <math>\angle ADB</math> and <math>\angle CDA</math> are [[supplementary]], <math>m\angle ADB = 180^\circ - m\angle CDA</math>.  We can therefore solve both equations for the cosine term.  Using the [[trigonometric identity]] <math>\cos{\theta} = -\cos{(180^\circ - \theta)}</math> gives us
+*<math>\frac{n^2 + d^2 - b^2}{2nd} = \cos{\angle CDA}</math>
+*<math>\frac{c^2 - m^2 -d^2}{2md} = \cos{\angle CDA}</math>
+Setting the two left-hand sides equal and clearing [[denominator]]s, we arrive at the equation: <math> c^{2}n + b^{2}m=m^{2}n +n^{2}m + d^{2}m + d^{2}n </math>. However, <math>m+n = a</math> so
+<cmath>m^2n + n^2m = (m + n)mn = amn</cmath> and
+<cmath>d^2m + d^2n = d^2(m + n) = d^2a.</cmath>
+This simplifies our equation to yield <math>man + dad = bmb + cnc,</math> as desired.
+=== Proof 2 (Pythagorean Theorem) ===
+Let the [[altitude]] from <math>A</math> to <math>BC</math> meet <math>BC</math> at <math>H</math>. Let <math>AH=h</math>, <math>CH=x</math>, and <math>HD=y</math>.
+We can apply the [[Pythagorean Theorem]] on <math>\triangle AHC</math> and <math>\triangle AHD</math> to yield <math>h^2 = b^2 - x^2 = d^2 - y^2</math>  and then solve for <math>b</math> to get  <math>b^2 = d^2 + x^2 - y^2</math>.
+Doing the same for <math>\triangle AHB</math> and <math>\triangle AHD</math> gives us:
+<cmath>h^2 = c^2 - (m + y)^2 = d^2 - y^2,</cmath> then we solve for <math>c</math> to get <math>c^2 = d^2 + m^2 + 2my</math>.
+Now multiple the first expression by <math>m</math> and the second by <math>n</math>:
+\begin{align*}
+mb^2 &= md^2 + m(x^2 - y^2)\\
+nc^2 &= nd^2 + m^2n + 2mny
+\end{align*}
+Next, we add these two expressions:
+<cmath>mb^2 + nc^2 = md^2 + m(x^2 - y^2) + nd^2 + m^2n + 2mny.</cmath>
+Then simplify as follows (we reapply <math>x + y = n</math> a few times while factoring):
+\begin{align*}
+mb^2 + nc^2 &= (m + n)d^2 + m(x+y)(x - y) + mn(n + 2y)\\
+&= (m + n)d^2 + mn(x - y) + mn(n + 2y)\\
+&= (m + n)d^2 + mn(x + y + n)\\
+&= (m + n)d^2 + mn(m + n)\\
+&= (m + n)(d^2 + mn).
+\end{align*}
+Rearranging the equation gives Stewart's Theorem: <math>man+dad = bmb+cnc</math>.
+== Proof 3 (Barycentrics) ==
+Let the following points have the following coordinates:
+<math>A: (1,0,0)</math>
+<math>B: (0,1,0)</math>
+<math>C: (0,0,1)</math>
+<math>D: \left(0, \frac{n}{m+n},\frac{m}{m+n}\right)</math>
+Our displacement vector <math>\overrightarrow{AD}</math> has coordinates <math>\left(1, -\frac{n}{m+n}, -\frac{m}{m+n}\right)</math>. Plugging this into the barycentric distance formula, we obtain <cmath>d^2=-(m+n)^2 \left(\frac{mn}{(m+n)^2} \right)-b^2 \left ( -\frac{m}{m+n} \right)-c^2 \left(-\frac{n}{m+n}\right)=-mn+\frac{b^2m+c^2n}{m+n}</cmath> Multiplying by <math>m+n</math>, we get <math>d^2(m+n)+mn(m+n)=b^2m+c^2n</math>. Substituting <math>m+n</math> with <math>a</math>, we find Stewart's Theorem: <cmath>\boxed{d^2a+amn=b^2m+c^2n}</cmath>
+== Video Proof ==
-Now we set the two equal and arrive at Stewart's theorem: <math> c^{2}n + b^{2}m=m^{2}n +n^{2}m + t^{2}m + t^{2}n </math>
+[//youtu.be/jEVMgWKQIW8 TheBeautyofMath]
+== See Also ==
-== Example ==
+* [[Menelaus' theorem]]
-''(awaiting addition)''
+* [[Ceva's theorem]]
+* [[Geometry]]
+* [[Angle Bisector Theorem]]
-== See also ==
+[[Category:Geometry]]
-* [[Menelaus' Theorem]]
+[[Category:Theorems]]
-* [[Ceva's Theorem]]
+{{stub}}

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