Heron's formula relates the area, A, of a triangle with the half perimeter, s: [1.1] where s=(a+b+c)/2, and a, b, c are the lengths of the sides.

Where
the only information we have about a triangle is the length of its
sides, Heron's formula is appropriate to use to compute the area.

Proof

The following proof is trigonometric, and basically uses the cosine rule.
First we compute the cosine squared in terms of the sides, and then the
sine squared which we use in the formula A=1/2bc·sinA
to derive the area of the triangle in terms of its sides, and thus prove Heron's formula.

We use the relationship x^{2}−y^{2}=(x+y)(x−y) [difference between two squares] [1.2]

Finding the cosine squared in terms of the sides

From the cosine rule:

We have: [1.3]

Rearranging: [1.4]

Because we want the sine, we first square the cosine: [1.5]

Finding the Sine

To use in: [1.6]

Using Equation 1.5 in 1.6, we have: [1.7]

Bringing all under the same denominator: [1.8]

Using the difference between two squares (Equation 1.2) [1.9]

Putting the above into a form where we can use the difference between two squares again we have: [1.10] Actually using the difference between two squares in both brackets, we find:

[1.11]

Substituting (a+b+c) for 2s, (b+c-a) for 2s-2a, etc: [1.12]

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