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6.1: Analyzing Heron’s Formula

Difficulty Level: At Grade Created by: CK-12
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This activity is intended to supplement Trigonometry, Chapter 5, Lesson 2.

Time Required: 20 minutes

Activity Overview

In this activity, students will use their graphing calculators to determine the relationship between Heron’s Formula and the basic area formula.

Topics Covered

  • Finding the area of a triangle
  • Points of intersection
  • Interpreting a graph

Teacher Preparation and Notes

  • Make sure students have cleared \begin{align*}Y=\end{align*}Y= menu before starting.
  • You may need to remind students how to TRACE and find points of intersection.

Associated Materials

Problem 1: The 3, 4, 5 right triangle

  • Students should know this is a right triangle, with hypotenuse 5. Make sure that they have that the legs are 3 and 4.
  • The area of this triangle is 6.
  • With Heron’s Formula, \begin{align*}A = \sqrt{s(s - a)(s - b)(s - c)}\end{align*}A=s(sa)(sb)(sc), and \begin{align*}Y1 = \sqrt{x(x - 3)(x - 4)(x - 5)}\end{align*}Y1=x(x3)(x4)(x5) students might get confused with the parenthesis. Make sure all students change their window to the dimensions to the right before graphing.


\begin{align*}Xmin = -1\end{align*}Xmin=1

\begin{align*}Xmax = 8\end{align*}Xmax=8

\begin{align*}Xscl = 1\end{align*}Xscl=1

\begin{align*}Ymin = -1\end{align*}Ymin=1

\begin{align*}Ymax = 10\end{align*}Ymax=10

\begin{align*}Yscl = 1\end{align*}Yscl=1

\begin{align*}Xres = 1\end{align*}Xres=1

  • The graph is to the right. Have students analyze the domain and range and why there are blank spaces in the graph. The domain is \begin{align*}(\infty, 00]\end{align*}(,00], [3, 4], and \begin{align*}[5, \infty)\end{align*}[5,) and the range is all real numbers greater than zero. If you have students zoom in further, they will see that there are no \begin{align*}x-\end{align*}xintercepts and one \begin{align*}y\end{align*}y intercept at (0, 0).

  • \begin{align*}Y2= 6\end{align*}Y2=6 represents the area of this triangle. The horizontal line crosses the graph at (-0.435, 6) and (6, 6). The first point, however is invalid because \begin{align*}x\end{align*}x cannot be negative here. Ask students why. Explain that \begin{align*}x\end{align*}x is actually \begin{align*}s\end{align*}s and that \begin{align*}s\end{align*}s cannot be negative, because by definition it is \begin{align*}\frac{1}{2}(a + b + c)\end{align*}12(a+b+c).
  • Specifically, the point (6, 6) represents (\begin{align*}s\end{align*}s, area). So, from this system of equations, we have determined what \begin{align*}s\end{align*}s is such that we have the correct area, in this case, also 6. If we can find the area in more that one manner, this will always work as a way to solve for \begin{align*}s\end{align*}s.

Problem 2

For this triangle, the area is, \begin{align*}A = \frac{1}{2}(15)\left(\frac{4 \sqrt{17}}{3} \right) = 5 \cdot 2 \sqrt{17} = 10 \sqrt{17} \approx 41\end{align*}

Here, students would graph \begin{align*}y = \sqrt{x(x - 7)(x - 12)(x - 15)}\end{align*} and \begin{align*}y = 10 \sqrt{17}\end{align*} to determine what \begin{align*}x\end{align*}, or \begin{align*}s\end{align*}, is. The graph of the two functions looks like:

Again, \begin{align*}s\end{align*} cannot be negative, so we eliminate the negative point of intersection. Therefore the answer is \begin{align*}\left(17, 10 \sqrt{17}\right)\end{align*}.

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Date Created:
Feb 23, 2012
Last Modified:
Nov 04, 2014
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