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.
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 and point . We will call the distance between and .
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, .
Distance Formula: The distance between and is .
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 and , if , 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 and , if , then the triangle is obtuse.
What is the height of the isosceles triangle?
Draw the altitude from the vertex between the congruent sides, which will bisect the base.
Find the distance between (1, 5) and (5, 2).
Make and . Plug into the distance formula.
Just like the lengths of the sides of a triangle, distances are always positive.
Graph , and . Determine if is acute, obtuse, or right.
Use the distance formula to find the length of each side.
Plug these lengths into the Pythagorean Theorem.
is an obtuse triangle.
Determine if the following triangles are acute, right or obtuse.
3. A triangle with side lengths 5, 12, 13.
Set the longest side equal to .
The triangle is acute.
The triangle is obtuse.
3. so this triangle is right.
Find the height of each isosceles triangle below. Simplify all radicals.
Find the length between each pair of points.
- (-1, 6) and (7, 2)
- (10, -3) and (-12, -6)
- (1, 3) and (-8, 16)
- What are the length and width of a 42” HDTV? Round your answer to the nearest tenth.
- 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.
- 7, 8, 9
- 14, 48, 50
- 5, 12, 15
- 13, 84, 85
- 20, 20, 24
- 35, 40, 51
- 39, 80, 89
- 20, 21, 38
- 48, 55, 76
Graph each set of points and determine whether is acute, right, or obtuse, using the distance formula.