Difference between revisions of "Heron's Formula"

Revision as of 15:52, 23 February 2025

Theorem

In geometry, Heron's formula (or Hero's formula) gives the area of a triangle in terms of three side lengths, $a$ , $b$ , $c$ . Letting $s$ be the semiperimeter of the triangle, $s = \frac{1}{2} (a + b + c)$ , the area $A$ is

$A=\sqrt{s(s-a)(s-b)(s-c)}$

It is named after the first-century engineer Heron of Alexandria (or Hero) who proved it in his work Metrica, though it was probably known centuries earlier.

Proof

Using basic Trigonometry, we have $[ABC]=\frac{ab}{2}\sin C,$ which simplifies to $[ABC]=\frac{ab}{2}\sqrt{1-\cos^2 C}.$

The Law of Cosines states that in triangle $ABC$ , $c^2 = a^2 + b^2 - 2ab\cos C$ , which can be written as $\cos C = \frac{a^2 + b^2 - c^2}{2ab}$ . Thus, $[ABC]=\frac{ab}{2}\sqrt{1-\left(\frac{a^2+b^2-c^2}{2ab}\right)^2}.$

Now, we can simplify: \begin{align*} [ABC] &= \sqrt{\frac{a^2b^2}{4} \left( 1 - \frac{(a^2 + b^2 - c^2)^2}{4a^2b^2} \right)} \\ &= \sqrt{\frac{4a^2b^2 - (a^2 + b^2 - c^2)^2}{16}} \\ &= \sqrt{\frac{(2ab + a^2 + b^2 - c^2)(2ab - a^2 - b^2 + c^2)}{16}} \\ &= \sqrt{\frac{((a + b)^2 - c^2)((a - b)^2 - c^2)}{16}} \\ &= \sqrt{\frac{(a + b + c)(a + b - c)(b + c - a)(a + c - b)}{16}} \\ &= \sqrt{s(s - a)(s - b)(s - c)} \end{align*}

Isosceles Triangle Simplification

$A=\sqrt{s(s-a)(s-b)(s-c)}$ for all triangles

$b=c$ for all isosceles triangles

$A=\sqrt{s(s-a)(s-b)(s-b)}$ simplifies to $A=(s-b)\sqrt{s(s-a)}$

Square root simplification/modification

From $A=\sqrt{s(s-a)(s-b)(s-c)}$ We can "take out" the $1/2$ in each $s$ , then we have $A=\frac{1}{4}\sqrt{(a+b+c)(-a+b+c)(a-b+c)(a+b-c)}$ Using the difference of squares on the first two and last two factors, we get $A=\frac{1}{4}\sqrt{(b^2+2bc+c^2-a^2)(a^2-b^2+2bc-c^2)}$ and using the difference of squares again, we get $A=\frac{1}{4}\sqrt{(2bc)^2-(-a^2+b^2+c^2)^2}$ From this equation (although seemingly not symmetrical), it is much easier to calculate the area of a certain triangle with side lengths with quantities with square roots. One can remember this formula by noticing that when finding the cosine of an angle in a triangle, the formula is $\cos{A}=\frac{-a^2+b^2+c^2}{2bc}$ and the two terms in the formula are just the denominator and numerator of the fraction for $\cos{A}$ , only they're squared. This can also serve as a reason for why the area $A$ is never imaginary. This is equivalent of ending at step $4$ in the proof and distributing.

Note

Replacing $-a^2+b^2+c^2$ as $2bc\cos{A}$ , the area simplifies down to $\frac{1}{4}\sqrt{(2bc\sin{A})^2}$ , or $\frac{1}{4}\cdot2bc\sin{A}$ , or $\frac{1}{2}bc\sin{A}$ , another common area formula for the triangle.

Example

Let's say that you have a right triangle with the sides $3$ , $4$ , and $5$ . Your semi- perimeter would be $6$ since $(3+4+5)$ ÷ $2$ is $6$ . Then you have $6-3=3$ , $6-4=2$ , $6-5=1$ . $1\cdot 2\cdot 3=6.$ $6\cdot 6 = 36$ The square root of $36$ is $6$ . The area of your triangle is $6$ .

External Links

In general, it is a good advice not to use Heron's formula in computer programs whenever we can avoid it. For example, whenever vertex coordinates are known, vector product is a much better alternative. Main reasons:

Computing the square root is much slower than multiplication.
For triangles with area close to zero Heron's formula computed using floating point variables suffers from precision problems.

@@ Line 1: / Line 1: @@
-'''Heron's Formula''' (sometimes called Hero's formula) is a [[mathematical formula | formula]] for finding the [[area]] of a [[triangle]] given only the three side lengths.
+==Theorem==
-== Theorem ==
+In [[Geometry|geometry]], '''Heron's formula''' (or '''Hero's formula''') gives the [[Area|area]] of a [[Triangle|triangle]] in terms of three side lengths, <math>a</math>, <math>b</math>, <math>c</math>. Letting <math>s</math> be the [[Semi-perimeter|semiperimeter]] of the triangle, <math>s = \frac{1}{2} (a + b + c)</math>, the area <math>A</math> is
-For any triangle with side lengths <math>{a}, {b}, {c}</math>, the area <math>{A}</math> can be found using the following formula:
+<cmath>A=\sqrt{s(s-a)(s-b)(s-c)}</cmath>
-<math>A=\sqrt{s(s-a)(s-b)(s-c)}</math>
+It is named after the first-century engineer [https://en.wikipedia.org/wiki/Heron_of_Alexandria Heron of Alexandria] (or Hero) who proved it in his work ''Metrica'', though it was probably known centuries earlier.
-where the [[semi-perimeter]] <math>s=\frac{a+b+c}{2}</math>.
 == Proof ==

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