EMAT 6690
Heron’s Formula
Ken Montgomery
Although the area formula for a triangle enables one to easily calculate the area of a triangular region, one must be able to measure the height (Equation 1).
Equation
1:
In some cases, as in a farmer’s field for instance, a measure of the height is difficult to obtain, although the individual side lengths are easily measured (Figure 1).
Figure 1: with side lengths, a, b and c
A calculation of area, given the individual side lengths is readily made with use of Heron’s formula (Equation 2), where s, is the semi-perimeter, and is equal to one-half of the Perimeter (Figure 1).
Equation 2:
Equation 3 represents one of the three relations from the Law of Cosines.
Equation 3:
Solving Equation 1 for cos( A), we obtain Equation 4.
Equation 4:
Equation 5 represents a fundamental trigonometric identity:
Equation
5:
Solving Equation 3 for , we obtain Equation 6.
Equation
6:
Substituting from Equation 4, we have Equation 7.
Equation
7:
Squaring the term in parenthesis, we obtain Equation 8.
Equation
8:
Rewriting 1 with a common denominator gives Equation 9.
Equation
9:
Distributing the minus sign yields Equation 10.
Equation
10:
Combining like-terms and factoring yields Equation 11.
Equation
11:
Taking the square root of both sides gives Equation 12.
Equation
12:
The area formula for a triangle is given in Equation 13.
Equation 13:
In figure 2, we have , with height h and base, b, partitioned into segments and.
Figure
2: with
height, h and partitioned base, b
From Figure 2 and trigonometric ratio of sides, we obtain Equation 14.
Equation
14:
Substitution from Equation 14 into Equation 13 yields Equation 15.
Equation
15:
Substituting from Equation 12, we obtain Equation 16.
Equation
16:
Simplifying yields Equation 17.
Equation
17:
Distributing and rearranging terms yields Equation 18.
Equation
18:
We then write the equivalent relation given in Equation 19.
Equation
19:
Rearranging terms yields Equation 20.
Equation
20:
We rewrite one of the terms in the discriminate to obtain Equation 21.
Equation
21:
We add zero to the discriminate via canceling terms, obtaining Equation 22.
Equation
22:
We then factor the discriminate in Equation 22, yielding the simpler Equation 23.
Equation
23:
We add zero again, via canceling terms, to the two factors of the discriminate obtaining Equation 24.
Equation 24:
These additions of zero allow for the factoring of the discriminate in Equation 24, yielding the more elegant Equation 25.
Equation
25:
Distributing the fraction under the radical, we obtain Equation 26.
Equation
26:
We factor out the one-half from each factor of the radical obtaining Equation 27.
Equation
27:
We define semi-perimeter (s) to be half of the perimeter in Equation 28.
Equation
28:
Substitution from Equation 28 allows for the familiar expression of Heron’s formula in Equation 29.
Equation
29:
Click here to open HeronsFormula.gsp and explore the
relationship
between perimeter and area.
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