8.4 Right Triangle Applications
Bob Brown; Morgan Chase; and Marilyn Nielson
Applications
Here’s an age-old forestry problem: How tall is this tree?
The Hendricks Creek survivor elm stands on lands managed by the Wisconsin Board of Commissioners of Public Lands in northern Wisconsin. USDA Forest Service photo by Linda Haugen.
What do you need to know to find the height of this tree? How could you collect that data? What tools would you need?
Gaps
You’re on one side of a river (or a canyon, or a swampy muskeg, or a busy highway) and need to know the distance to the other side. Right triangles can help and it’s like surveying magic!
You’ll need to know one of the legs of the triangle and one of the angles. Which ones can you readily measure?
Use a tape to measure the distance you walk from the object you sighted on the other side to the point you’ll sight back to the object (side b). Use a compass to measure angle α. Viola! You have an angle and a side of a right triangle. Use these to find side a.
(In this particular diagram, you’d need to subtract the distance you stood from the river’s edge from a to get the width of the river.)
[latex]\text{tan }\alpha= \frac{a}{b}[/latex]
so
b \cdot \text{tan }\alpha= a
In general, we call side b the baseline and angle α is our “angle to point”.
Examples
Tree Heights
Check out the sweet triangle we can make with a tree! Which parameters can we measure easily in the field?
On flat ground, if you’ve got the angle to the of the tree and you know your distance from the tree and your height at eye level – you can build a straightforward triangle.
[latex](\text{tan (angle to top of tree)} \cdot \text{distance from tree}) + \text{your height at eye level} = \text{height of tree}[/latex]
We’ve got a general formula that will work with trees on a slope too, but let’s take some time to look at slope first!
Examples
A) You are standing 66 feet from a tree on level ground. Your eye is at 5’6″. The angle to the top of the tree is 12°. How tall is this tree?
B) You are standing 100 meters from a tree on level ground. Your eye is 171 cm tall. The angle to the top of the tree is 8°. How tall is this tree?
Slope
The slope of a surface is a measure of its steepness. In some cases, such as a walkway or ramp or street, a shallow slope is safer than a steep slope. In other cases, such as a roof, a steep slope may be preferred because it allows rainwater or accumulated snow to move off the roof surface more easily than a shallow slope.[1]
We will first look at slope in terms of vertical and horizontal distances, and we will then look at slope in terms of angles. Come on, let’s hit the slopes!
Slope as a Ratio
Slope is defined as the ratio of the vertical rise to the horizontal run.
- A line that is increasing in height has a positive slope.
- A line that is decreasing in height has a negative slope.[2]
- The slope of a horizontal line is 0; it is not increasing or decreasing.
A slope may be expressed as a ratio, a decimal, or a percent grade. For example, consider this loading ramp with a vertical rise of 23.5 inches and a horizontal run of 132 inches.
As a ratio, the slope is 23.5 : 132. Dividing the rise by the run gives a decimal value of approximately 0.18. Moving the decimal point two places to the right gives a grade of approximately 18%.
Check out Forest Measurements by Joan DeYoung. It has an excellent discussion of slope and its measurement and applications in the field of forestry.
Practice Exercises
Express each slope in three ways: as a ratio, a decimal, and a percent grade.
Slope as an Angle
The steepness of a line may also be described by its angle of elevation above the horizontal (or its angle of depression below the horizontal).
Notice that the vertical rise is the side opposite the angle, and the horizontal run is the side adjacent to the angle. Therefore, trigonometry tells us that [latex]\text{tangent}=\frac{\text{opposite}}{\text{adjacent}}=\frac{\text{rise}}{\text{run}}[/latex]. The tangent of the angle is equal to the slope.
Or, thinking about it in reverse, the inverse tangent of the slope is the angle.
[latex]\text{tan}^{-1}\left(\frac{\text{rise}}{\text{run}}\right)=\theta[/latex]
When we are looking at slope as a right triangle, the following terms will be synonymous:
Run = Horizontal Distance (HD) (also the distance if calculated from a map)
Rise = Vertical Distance (VD) (you can get this by counting contour lines on a topo map!)
Distance along the Hypotenuse = Slope Distance (SD)
Examples
A) SD = 124′ Slope Angle = 5° Find HD and VD.
B) Slope Angle = 15° HD = 125′ Find SD and VD.
C) HD = 120′ VD = -12′ Find SD and Slope Angle.
D) You’re on a slope of 12° and measure along the ground a distance of 100 feet (SD). How far is this as a horizontal distance?
E) A slope rises 10 feet for every 150 horizontal feet. What is the slope angle in degrees?
F) On a map, you place a proposed 400-yard fenceline. The fence runs directly up a slope that is 8°. What will the actual length of the fenceline be on the ground?
G) Two points on a map are 4 inches apart. The map scale is 1:24000. The starting elevation is 1800’and the endpoint elevation is 2200′.
- What is the distance between the two points on the ground (not accounting for slope).
- What is the slope?
- What will the actual distance be if measured on the ground?
Practice Exercises
For exercises 5 through 8, determine the angle of elevation for each slope. Round to the nearest hundredth of a degree.
For proper drainage, the ground around a building should slope downwards, away from the building.
9. The minimum downward grade of the ground is 1%. Assuming this grade, if a point on the ground is 50 feet horizontally from the base of the house, how much lower is the ground at that point?
10. The preferred minimum downward grade of the ground is 2%. Assuming this grade, if a point on the ground is 50 feet horizontally from the base of the house, how much lower is the ground at that point?
11. The maximum acceptable downward grade of the ground is 10%. Assuming this grade, if a point on the ground is 50 feet horizontally from the base of the house, how much lower is the ground at that point?
12. A motion sensor needs to be installed on the outside of a warehouse door 12 feet above the ground. The sensor should go off if anyone approaching the warehouse gets within 20 feet of the door. What angle from vertical should the sensor point away from the building to detect someone at the appropriate distance from the warehouse? Round your answer to the nearest degree.
13. A ramp is being constructed to the entrance to a public building that is 2.5 feet higher than the level courtyard in front of the entrance. Assuming that the ramp will be continuous with no switchbacks or level platforms,[5] what is the minimTum horizontal distance required for the ramp so that it will comply with ADA regulations?
Tree Height + Slope
When we measure trees in the field, we’ll usually be standing on slope (at least in this part of the world!). The distance that we measure by pulling a tape or pacing chains will be a slope distance (SD). We can get the slope angle with a clinometer. We can also get the angle to the top of a tree and the angle to the bottom of a tree with a clinometer.
How can we put these measurements together to arrive at the tree’s height?
Our first step is the get our horizontal distance from the tree. We can derive this from the slope distance (that we measured by pacing or pulling a tape/chain) and the slope angle (that we measured using a clinometer or similar tool).
[latex]\text{Horizontal Distance} = \text{Slope Distance}\cdot \text{Cos}(\text{slope angle})[/latex]
Then, we can calculate the total height of the tree by using the angles we measured to its top and bottom.
[latex]\text{Tree Height}= \text{Horizontal Distance}\cdot (\text{Tan angle to top - Tan angle to bottom})[/latex]
Where does the nifty formula for tree height come from?
Derive formula here
In this class, our work with tree height is limited to applications of trigonometric functions. We can also use percent slope (% slope) for the same calculations. Forest Measurements by Joan DeYoung has a great discussion and explanation of the arithmetic for working with percent slope rather than slope angle.
- Source: https://www.nachi.org/roof-slope-pitch.htm ↵
- We won't worry about negative slopes in this textbook, because we can always express a negative slope using a word like "decrease", "decline", or "depression" in combination with a positive number. ↵
- Source: https://www.ada-compliance.com/ada-compliance/403-walking-surfaces ↵
- Source: https://www.ada-compliance.com/ada-compliance/405-ramps ↵
- If you were curious, this is the maximum rise allowed for this situation; a rise of more than 2.5 feet would require a switchback or a level platform. ↵
Applications
You're a wildlife technician working on a behavioral study of Oregon spotted frogs (Rana pretiosa). You are looking at the time required for larvae to metamorphose into adults and the reaction time of adult frogs to prey items under a variety of conditions in the lab. What units of time will you use to measure the development of larvae into adult frogs? What units of time will you use to measure an adult frog's reaction time? Are these units of measurement different? Why?
You're measuring stream habitat using a meter tape. Do you record measurements to the nearest meter? The nearest centimeter? The nearest millimeter? If you record your measurements to centimeters, you might measure a riffle section as 5.38 meters long and 1.46 meters wide. Should you report the area of this riffle as 7.8548 square meters?
Objectives
When you've finished this section, you will be able to perform the following:
- Choose a level of precision for applied math calculations.
- Round to that level of precision.
In the first few modules, we rarely concerned ourselves with rounding; we assumed that every number we were told was exact and we didn't have to worry about any measurement error. However, every measurement contains some error. A standard sheet of paper is [latex]8.5[/latex] inches wide and [latex]11[/latex] inches high, but it's possible that the actual measurements could be closer to [latex]8.4999[/latex] and [latex]11.0001[/latex] inches. Even if we measure something very carefully, with very sensitive instruments, we should assume that there could be some small measurement error.[1]
Exact Values and Approximations
A number is an exact value if it is the result of counting or a definition.
A number is an approximation if it is the result of a measurement or of rounding.
Exercises
Identify each number as an exact value or an approximation.
- An inch is [latex]\frac{1}{12}[/latex] of a foot.
- This board is [latex]78[/latex] inches long.
- There are [latex]14[/latex] students in class.
- A car’s tachometer reads [latex]3,000[/latex] rpm.
- A right angle measures [latex]90^\circ[/latex].
- The angle of elevation of a ramp is [latex]4^\circ[/latex].
Accuracy and Significant Figures
Because measurements are inexact, we need to consider how accurate they are. This requires us to think about significant figures—often abbreviated "sig figs" in conversation—which are the digits in the measurement that we trust to be correct. The accuracy of a number is equal to the number of significant figures. (By the way, the terms "significant digits" and "significant figures" are used interchangeably.) The following rules aren't particularly difficult to understand but they can take time to absorb and internalize, so we'll include lots of examples and exercises.
Significant Figures
- All nonzero digits are significant.
Ex: [latex]12,345[/latex] has five sig figs, and [latex]123.45[/latex] has five sig figs. - All zeros between other nonzero digits are significant.
Ex: [latex]10,045[/latex] has five sig figs, and [latex]100.45[/latex] has five sig figs. - Any zeros to the right of a decimal number are significant.
Ex: [latex]123[/latex] has three sig figs, but [latex]123.00[/latex] has five sig figs. - Zeros on the left of a decimal number are NOT significant.
Ex: [latex]0.123[/latex] has three sig figs, and [latex]0.00123[/latex] has three sig figs. - Zeros on the right of a whole number are NOT significant unless they are marked with an overbar.
Ex: [latex]12,300[/latex] has three sig figs, but [latex]12,30\overline{0}[/latex] has five sig figs.
Another way to think about #4 and #5 above is that zeros that are merely showing the place value—where the decimal point belongs—are NOT significant.
Exercises
Determine the accracy (i.e., the number of significant figures) of each number.
- [latex]63,400[/latex]
- [latex]63,040[/latex]
- [latex]63,004[/latex]
- [latex]0.085[/latex]
- [latex]0.0805[/latex]
- [latex]0.08050[/latex]
In 1856, the first official measurement of the height of Mount Everest—called Sagarmatha in Nepal and Chomolungma in Tibet—was announced. The height was determined to be exactly [latex]29,000[/latex] feet, but there was concern that people would think this was only a rough estimate rounded to the nearest thousand feet. Therefore, the height was announced as [latex]29,002[/latex] feet, so that everyone seeing that number would believe that the measurement was correct to the nearest foot.[2] Yes, to demonstrate the correctness of the measurement, an incorrect measurement was announced.
Instead of fudging a number like [latex]29,000[/latex] to show that it is correct to the nearest foot, we can write it with an an overbar to indicate that the zeros are significant. Putting [latex]29,00\overline{0}[/latex] in a newspaper headline in 1856 would probably have confused people, but you can handle it because you're in a math class. Writing [latex]29,00\overline{0}[/latex] is our way of saying "Really, to the nearest foot, it's exactly [latex]29,000[/latex] feet!"
Exercises
Determine the accuracy (i.e., the number of significant figures) of each number.
- [latex]29,000[/latex]
- [latex]29,\overline{0}00[/latex]
- [latex]29,0\overline{0}0[/latex]
- [latex]29,00\overline{0}[/latex]
Two things to remember: we don't put an overbar over a nonzero digit, and we don't need an overbar for any zeros on the right of a decimal number because those are already understood to be significant.
Accuracy-Based Rounding
As we saw in Chapter 0.4, it is often necessary to round a number. We often round to a certain place value, such as the nearest hundredth, but there is another way to round. Accuracy-based rounding considers the number of significant figures rather than the place value.
Accuracy-based rounding:
- Locate the rounding digit to which you are rounding by counting from the left until you have the correct number of significant figures.
- Look at the test digit directly to the right of the rounding digit.
- If the test digit is 5 or greater, increase the rounding digit by 1 and drop all digits to its right. If the test digit is less than 5, keep the rounding digit the same and drop all digits to its right.
Exercises
Round each number so that it has the indicated number of significant figures.
- [latex]51,837[/latex] (three sig figs)
- [latex]51,837[/latex] (four sig figs)
- [latex]4.2782[/latex] (two sig figs)
- [latex]4.2782[/latex] (three sig figs)
When the rounding digit of a whole number is a [latex]9[/latex] that gets rounded up to a [latex]0[/latex], we must write an overbar above that [latex]0[/latex].
Similarly, when the rounding digit of a decimal number is a [latex]9[/latex] that gets rounded up to a [latex]0[/latex], we must include the [latex]0[/latex] in that decimal place.
Exercises
Round each number so that it has the indicated number of significant figures. Be sure to include trailing zeros or an overbar if necessary.
- [latex]13,997[/latex] (two sig figs)
- [latex]13,997[/latex] (three sig figs)
- [latex]2.596[/latex] (two sig figs)
- [latex]2.596[/latex] (three sig figs)
The height of Mount Everest has changed over the years due to plate tectonics and earthquakes. In December 2020, it was jointly announced by Nepal and China that the summit of Mount Everest has an elevation of [latex]29,031.69[/latex] ft.[3]
- Round [latex]29,031.69[/latex] ft to two sig figs.
- Round [latex]29,031.69[/latex] ft to three sig figs.
- Round [latex]29,031.69[/latex] ft to four sig figs.
- Round [latex]29,031.69[/latex] ft to five sig figs.
- Round [latex]29,031.69[/latex] ft to six sig figs.
Accuracy when Multiplying and Dividing
Suppose you needed to square the number [latex]3\frac{1}{3}[/latex]. You could rewrite [latex]3\frac{1}{3}[/latex] as the improper fraction [latex]\frac{10}{3}[/latex] and then figure out that [latex](\frac{10}{3})^2 = \frac{100}{9}[/latex], which equals the repeating decimal [latex]11.111...[/latex]
Because most people prefer decimals to fractions, we might decide to round [latex]3\frac{1}{3}[/latex] to [latex]3.33[/latex] and find that [latex]3.33^2=11.0889[/latex]. The answer [latex]11.0889[/latex] looks very accurate, but it is a false accuracy because there is round-off error involved. Only when we round to three sig figs do we get an accurate result: [latex]11.0889[/latex] rounded to three sig figs is [latex]11.1[/latex], which is accurate because [latex]11.111...[/latex] rounded to three sig figs is also [latex]11.1[/latex]. It turns out that because [latex]3.33[/latex] has only three significant figures, our answer must be rounded to three significant figures.
Don't round off the original numbers; do the necessary calculations first, then round the answer as your last step.
Exercises
Use a calculator to multiply or divide as indicated. Then round to the appropriate level of accuracy.
- [latex]8.75\cdot12.25[/latex]
- [latex]355.12\cdot1.8[/latex]
- [latex]77.3\div5.375[/latex]
- [latex]53.2\div4.5[/latex]
- Suppose you are filling a 5-gallon can of gasoline. The gasoline costs [latex]\textdollar4.579[/latex] per gallon, and you estimate that you will buy [latex]5.0[/latex] gallons. How much should you expect to spend?
Bonus material: Here is a comic strip from xkcd.com showing that including a lot of decimal digits can give a false sense of accuracy.
An instrument used for measuring angles of slope, elevation, or depression of an object.
https://www.forestry-suppliers.com/Images/Original/1316_43895_p1.jpg
