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Use similar triangles to solve Exercises 37-38. A person who is 5 feet tall is standing 80 feet from the base of a tree and the tree casts an 86 -foot shadow. The person's shadow is 6 feet in length. What is the tree's height?

Short Answer

Expert verified
The height of the tree is approximately 71.67 feet.

Step by step solution

01

Setup the Similar Triangles

First, set up the proportion. The height of the person (5 feet) is to their shadow's length (6 feet) as the height of the tree is to its shadow's length (86 feet). It can be denoted as \( \frac{5}{6} = \frac{X}{86} \), where 'X' is the height of the tree.
02

Solving the Proportion

To find 'X', perform the cross-multiplication, it gives: \( X = \frac{5}{6} * 86 \).
03

Calculating the Tree's Height

Computing the right side of the above equation gives the height of the tree. So, multiply above values which gives 'X' approximately equal to 71.67 feet.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Proportion in Similar Triangles
When we have two triangles that are similar, their corresponding angles are equal and the ratios of their corresponding sides are proportional. This means that the lengths of the sides of one triangle are scaled versions of the lengths of the sides of the other triangle.

For instance, in the problem mentioned, we have two sets of similar triangles: the first set includes the person and their shadow, and the second set includes the tree and its shadow. Each set forms a right-angled triangle, where the person and the tree are the heights, and their shadows correspond to the bases of their respective triangles.

By establishing a proportion between the two sets of corresponding sides (person's height to their shadow and tree's height to its shadow), we create a proportion that allows us to solve for the unknown height of the tree. This method relies on the fundamental idea that similar triangles maintain the same shape but their size can be different. Using the given numerical values, we get the proportion:

\( \frac{5 \text{ feet}}{6 \text{ feet}} = \frac{X}{86 \text{ feet}} \)

Here, we're using the fact that the ratio between the person's height and their shadow's length is the same as the ratio between the tree's height and its shadow's length.
Cross-Multiplication
Cross-multiplication is a technique for solving proportions. It involves multiplying across the equality of two ratios, that is, multiplying the numerator of one ratio by the denominator of the other ratio. This process eliminates the fractions and makes it easier to solve for the unknown variable.

In this specific exercise, after setting up the proportion, cross-multiplication helps us solve for 'X', the tree's height. We multiply the person's height (5 feet) by the length of the tree's shadow (86 feet), and the person's shadow length (6 feet) by the unknown tree height 'X'.

The equation then becomes:

\( 5 \times 86 = 6 \times X \) \( 430 = 6X \) \( X = \frac{430}{6} \) \( X \text{ (tree's height) } \text{≈ } 71.67 \text{ feet} \)

It's an essential method for finding an unknown variable in proportional relationships and often comes in handy in geometry, especially when dealing with similar figures.
Shadow Length Comparison
Shadow length comparison in geometry problems often involves using similar triangles to relate the lengths of shadows to the heights of objects casting them. This is especially useful in real-world applications, such as determining the height of a tree, building, or other structures when direct measurement is not feasible.

The shadow cast by an object is determined by the angle of the light source, typically the sun. As long as the light source and ground are parallel, the angles created by the object and its shadow will be identical between two scenarios — for instance, between a person and a tree. As a result, we can say the triangles formed by the person and the tree with their respective shadows are similar.

By measuring the length of the person's shadow and knowing their height, we can set up a proportion to find the tree's height when we measure the length of the tree's shadow. This technique is quite resourceful as it uses the principle of similar triangles to obtain measurements indirectly and solve problems that would otherwise require complex measuring tools or calculations.

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