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## Praxis Core Math

### Unit 1: Lesson 5

Geometry- Properties of shapes | Lesson
- Properties of shapes | Worked example
- Angles | Lesson
- Angles | Worked example
- Congruence and similarity | Lesson
- Congruence and similarity | Worked example
- Circles | Lesson
- Circles | Worked example
- Perimeter, area, and volume | Lesson
- Perimeter, area, and volume | Worked example

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# Circles | Worked example

Sal Khan works through a question on circles from the Praxis Core Math test.

## Want to join the conversation?

- The lesson on Circle circumference was3:14long. Did you do that on purpose? ;)(2 votes)
- How did you know to find the laps instead of the miles, meaning how did you know to find that 1 lap is 1/6 of a mile instead of 1 mile is 6 laps.(1 vote)

## Video transcript

- [Narrator] So we have a question here that's dealing with circles. It says, "Ximena is running
laps around a circular track. If she runs a total distance
of 12 miles in 72 laps, what is the diameter,
in miles, of the track? So pause this video and
try to work through it on your own before we
work through it together. Okay, so let's do it together and so let's visualize what's going on. We have a track that is circular. That's my best freehand
drawing of a circle. Imagine an actual perfect circle. And we need to figure
out what the diameter of the circle is. So let's call that, that is
the diameter of the circle. And they give us some other information. She runs a total of 12 miles in 72 laps. So we should be able to
use this to figure out what is the circumference of the circle? The circumference of the circle would be how many miles she runs in one lap. So if we're able to figure out the circumference of the circle, then we can figure out
the diameter from that. How do we do that? Well, you might remember
that circumference is equal to the number
pi times the diameter. Or another way to think about it is, and this is actually where the
definition of pie comes from, circumference divided by diameter is defined to be the number pi. Or if you're solving for diameter you divide both sides of this by pi, and you get circumference divided by pi is equal to diameter. So if we can figure out circumference, we just divide by pi and
we're going to be able to answer this question. Now what's the circumference here? Well if 12 miles is equal to 72 laps, so 12 miles is equal to 72 laps, how do we figure out what
one lap is going to be? Well, we could divide both sides by 72. Because if we divide both sides by 72, so 72 here, we divide 72 over here, on the left hand side 12/72, well that's the same thing as 1/6. 12 divided by 12 is 1,
72 divided by 12 is 6. So we could say 1/6 of a mile, or 1/6 miles, but I'll
just say 1/6 of a mile, is equal to 72/72, that's
why we divided by 72. It's equal to one laps would
not be grammatically correct so I'll say it's equal to one lap. So just like that we've
been able to figure out the circumference of the circle. The circumference of the circle
is equal to 1/6 of a mile. So what's the diameter? Well it's going to be the
circumference divided by pi. So the diameter is going
to be the circumference, let me write this, it's
a mile, not a meter, so it's going to be 1/6 of a mile, which is the circumference divided by pi. Now that is the same
thing as 1/6 pi of a mile. And it's very atypical to
one, see circumferences that are fractions in most math problems, although of course in the real world they can be, and this is
also a very unusual distance right over here, but that is the diameter of the circumference of that track. So the diameter in miles
of the track is 1/6 pi. And we're done.