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# Distance Between Two Polar Coordinates

## Application of the law of cosines.

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Distance Between Two Polar Coordinates

When playing a game of darts with your friend, the darts you throw land in a pattern like this

You and your friend decide to find out how far it is between the two darts you threw. If you know the positions of each of the darts in polar coordinates, can you somehow find a formula to let you determine the distance between the two darts?

At the end of this Concept, you'll be able to answer this question.

### Guidance

Just like the Distance Formula for and coordinates, there is a way to find the distance between two polar coordinates. One way that we know how to find distance, or length, is the Law of Cosines, or . If we have two points and , we can easily substitute for and for . As for , it needs to be the angle between the two radii, or . Finally, is now distance and you have .

#### Example A

Find the distance between and .

Solution: After graphing these two points, we have a triangle. Using the new Polar Distance Formula, we have .

#### Example B

Find the distance between and .

Solution: This one is a little trickier than the last example because we have negatives. The first point would be plotted in the fourth quadrant and is equivalent to . The second point would be reflected across the pole, or . Use these two values of for the formula. Also, the radii should always be positive when put into the formula. That being said, the distance is .

#### Example C

Find the distance between and .

Solution: This problem is straightforward from looking at the relationship between the points. The two points lie at the same angle, so the straight line distance between them is . However, we can confirm this using the distance formula:

.

### Guided Practice

1. Given and , calculate the distance between the points.

and

2. Given and , calculate the distance between the points.

and

3. Given and , calculate the distance between the points.

and

Solutions:

1. Use .

2. Use .

3. Use .

### Concept Problem Solution

Using the Distance Formula for points in a polar plot, it is possible to determine the distance between the 2 darts:

### Explore More

Find the distance between each set of points.

1. and
2. and
3. and
4. and
5. and
6. and
7. and
8. and
9. and
10. and
11. Find the length of the arc between the points and .
12. Find the length of the arc between the points and .
13. Find the area of the sector created by the origin and the points and .
14. Find the area of the sector created by the origin and the points and .
15. Find the area of the sector created by the origin and the points and .