<meta http-equiv="refresh" content="1; url=/nojavascript/"> Applications of the Pythagorean Theorem ( Read ) | Geometry | CK-12 Foundation

# Applications of the Pythagorean Theorem

%
Best Score
Practice Applications of the Pythagorean Theorem
Best Score
%

# Applications of the Pythagorean Theorem

What if you were given an equilateral triangle in which all the sides measured 4 inches? How could you use the Pythagorean Theorem to find the triangle's altitude? After completing this Concept, you'll be able to solve problems like this one.

### Guidance

##### Find the Height of an Isosceles Triangle

One way to use The Pythagorean Theorem is to find the height of an isosceles triangle (see Example A).

##### Prove the Distance Formula

Another application of the Pythagorean Theorem is the Distance Formula. We will prove it here.

Let’s start with point $A(x_1, y_1)$ and point $B(x_2, y_2)$ . We will call the distance between $A$ and $B, d$ .

Draw the vertical and horizontal lengths to make a right triangle.

Now that we have a right triangle, we can use the Pythagorean Theorem to find the hypotenuse, $d$ .

$d^2 &= (x_1-x_2)^2 + (y_1-y_2)^2\\d &= \sqrt{(x_1 - x_2)^2 + (y_1 - y_2)^2}$

Distance Formula: The distance between $A(x_1, y_1)$ and $B(x_2, y_2)$ is $d = \sqrt{(x_1 - x_2)^2 + (y_1 - y_2)^2}$ .

##### Classify a Triangle as Acute, Right, or Obtuse

We can extend the converse of the Pythagorean Theorem to determine if a triangle is an obtuse or acute triangle.

Acute Triangles: If the sum of the squares of the two shorter sides in a right triangle is greater than the square of the longest side, then the triangle is acute.

For $b < c$ and $a < c$ , if $a^2 + b^2 > c^2$ , then the triangle is acute.

Obtuse Triangles: If the sum of the squares of the two shorter sides in a right triangle is less than the square of the longest side, then the triangle is obtuse.

For $b < c$ and $a < c$ , if $a^2+b^2 , then the triangle is obtuse.

#### Example A

What is the height of the isosceles triangle?

Draw the altitude from the vertex between the congruent sides, which will bisect the base.

$7^2 + h^2 &= 9^2\\49 + h^2 &= 81\\h^2 &= 32\\h &= \sqrt{32} = \sqrt{16 \cdot 2} = 4 \sqrt{2}$

#### Example B

Find the distance between (1, 5) and (5, 2).

Make $A(1, 5)$ and $B(5, 2)$ . Plug into the distance formula.

$d &= \sqrt{(1-5)^2 + (5-2)^2}\\&= \sqrt{(-4)^2 + (3)^2}\\&= \sqrt{16+9} = \sqrt{25} = 5$

Just like the lengths of the sides of a triangle, distances are always positive.

#### Example C

Graph $A(-4, 1), B(3, 8)$ , and $C(9, 6)$ . Determine if $\triangle ABC$ is acute, obtuse, or right.

Use the distance formula to find the length of each side.

$AB &= \sqrt{(-4-3)^2 + (1-8)^2} = \sqrt{49+49} = \sqrt{98}\\BC &= \sqrt{(3-9)^2 + (8-6)^2} = \sqrt{36 + 4} = \sqrt{40}\\AC &= \sqrt{(-4-9)^2 + (1-6)^2} = \sqrt{169 + 25} = \sqrt{194}$

Plug these lengths into the Pythagorean Theorem.

$\left( \sqrt{98} \right )^2 + \left( \sqrt{40} \right)^2 & \ ? \ \left ( \sqrt{194} \right )^2\\98 + 40 & \ ? \ 194\\138 & < 194$

$\triangle ABC$ is an obtuse triangle.

### Guided Practice

Determine if the following triangles are acute, right or obtuse.

1.

2.

3. A triangle with side lengths 5, 12, 13.

Set the longest side equal to $c$ .

1. $6^2 + \left( 3 \sqrt{5} \right)^2 & \ ? \ 8^2\\36 + 45 & \ ? \ 64\\81 & > 64$

The triangle is acute.

2. $15^2 + 14^2 & \ ? \ 21^2\\225 + 196 & \ ? \ 441\\421 & < 441$

The triangle is obtuse.

3. $5^2 +12^2 = 13^2$ so this triangle is right.

### Practice

Find the height of each isosceles triangle below. Simplify all radicals.

Find the length between each pair of points.

1. (-1, 6) and (7, 2)
2. (10, -3) and (-12, -6)
3. (1, 3) and (-8, 16)
4. What are the length and width of a 42” HDTV? Round your answer to the nearest tenth.
5. Standard definition TVs have a length and width ratio of 4:3. What are the length and width of a 42” Standard definition TV? Round your answer to the nearest tenth.

Determine whether the following triangles are acute, right or obtuse.

1. 7, 8, 9
2. 14, 48, 50
3. 5, 12, 15
4. 13, 84, 85
5. 20, 20, 24
6. 35, 40, 51
7. 39, 80, 89
8. 20, 21, 38
9. 48, 55, 76

Graph each set of points and determine whether $\triangle ABC$ is acute, right, or obtuse, using the distance formula.

1. $A(3, -5), B(-5, -8), C(-2, 7)$
2. $A(5, 3), B(2, -7), C(-1, 5)$
3. $A(1, 6) , B(5, 2), C(-2, 3)$
4. $A(-6, 1), B(-4, -5), C(5, -2)$