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1.4: Pythagorean Theorem to Determine Distance

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While walking to school one day, you decide to use your knowledge of the Pythagorean Theorem to determine how far it is between your home and school. You know that you walk 3 blocks east, and then turn and walk 7 blocks north to get to school. Is it possible to use the Pythagorean Theorem to help you determine the "straight line" distance between your home and school? By the end of this Concept, you'll be able to use the Pythagorean Theorem to do just that.

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James Sousa: Determine the Distance Between Two Points

Guidance

An application of the Pythagorean Theorem is to find the distance between two points. Consider the points (-1, 6) and (5, -3). If we plot these points on a grid and connect them, they make a diagonal line. Draw a vertical line down from (-1, 6) and a horizontal line to the left of (5, -3) to make a right triangle.

Now we can find the distance between these two points by using the vertical and horizontal distances that we determined from the graph.

9^2 + (-6)^2 & = d^2\\81 + 36 & = d^2\\117 & = d^2\\\sqrt{117} & = d\\3\sqrt{13} & = d

Notice, that the x- values were subtracted from each other to find the horizontal distance and the y- values were subtracted from each other to find the vertical distance. If this process is generalized for two points (x_1, \ y_1) and (x_2, \ y_2) , the Distance Formula is derived.

(x_1-x_2)^2 + (y_1-y_2)^2 = d^2

This is the Pythagorean Theorem with the vertical and horizontal differences between (x_1, \ y_1) and (x_2, \ y_2) . Taking the square root of both sides will solve the right hand side for d , the distance.

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

This is the Distance Formula. The following example shows how to apply the distance formula.

Example A

Find the distance between the two points.

(4, 2) and (-9, 5)

Solution: Plug each pair of points into the distance formula.

d & = \sqrt{(4-(-9))^2 + (2-5)^2}\\& = \sqrt{13^2 + (-3)^2}\\& = \sqrt{169+9}\\& = \sqrt{178}

Example B

Find the distance between the two points.

(-10, 3) and (0, -15)

Solution: Plug each pair of points into the distance formula.

d & = \sqrt{(-10-0)^2 + (3-(-15))^2}\\& = \sqrt{(-10)^2 + (18)^2}\\& = \sqrt{100+324}\\& = \sqrt{424} = 2\sqrt{106}

Example C

Find the distance between the two points.

(3, 1) and (2, -7)

Solution: Plug each pair of points into the distance formula.

d & = \sqrt{(3-2)^2 + (1-(-7))^2}\\& = \sqrt{(1)^2 + (8)^2}\\& = \sqrt{1+64}\\& = \sqrt{65}

Vocabulary

Distance Formula: The Distance Formula is a formula using the Pythagorean Theorem to find the distance between two points on a coordinate system using the differences between the "x" and "y" coordinates of the two points as side lengths.

Guided Practice

Find the distance between the two points.

1. (3, 1) and (2, -7)

2. (5, -8) and (0, 3)

3. (2, 6) and (2, 9)

Solutions:

1. Plug each pair of points into the distance formula.

d & = \sqrt{(3-2)^2 + (1-(-7))^2}\\& = \sqrt{(1)^2 + (8)^2}\\& = \sqrt{1+64}\\& = \sqrt{65}

2. Plug each pair of points into the distance formula.

d & = \sqrt{(5-0)^2 + (-8-(3))^2}\\& = \sqrt{(5)^2 + (-11)^2}\\& = \sqrt{25+121}\\& = \sqrt{146}

3. Plug each pair of points into the distance formula.

d & = \sqrt{(2-2)^2 + (6-9)^2}\\& = \sqrt{(0)^2 + (-3)^2}\\& = \sqrt{9} = 3

Concept Problem Solution

Since you know that the trip to school involves walking 3 blocks east followed by 7 blocks north, you can construct a triangle on a coordinate system out of these lengths, like this:

Since you went three blocks to the east, the school has an "x" coordinate of 3. Likewise, since you went 7 blocks north, the school has a "y" coordinate of 7. To find the straight-line distance to school, you can use the Distance Formula:

d & = \sqrt{(3-0)^2 + (7-0)^2}\\& = \sqrt{(3)^2 + (7)^2}\\& = \sqrt{58}

This is a straight line distance of approximately 7.6 blocks.

Explore More

Find the distance between each pair of points. Round each answer to the nearest tenth.

  1. (2, 4) and (5, 10)
  2. (1, 5) and (8, 9)
  3. (-2, 3) and (6, 4)
  4. (5, 7) and (5, 10)
  5. (8, 12) and (15, 12)
  6. (1, -4) and (25, -2)
  7. (5, -6) and (3, 7)
  8. (12, -9) and (-1, 5)
  9. (-3, 14) and (8, 10)
  10. (-11, 3) and (-5, 1)
  11. (5, 2) and (11, 13)
  12. (8, 10) and (9, -6)

Find the perimeter of each triangle. Round each answer to the nearest tenth.

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

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At Grade

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Date Created:

Sep 26, 2012

Last Modified:

Oct 28, 2014
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