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ACT Math Videos 164 videos

ACT Math 1.1 Pre-Algebra
1061 Views

Pre-Algebra Drill 1, Problem 1. Calculate 23 + 53(7 – 4).

ACT Math 1.1 Trigonometry
355 Views

ACT Math: Trigonometry Drill 1, Problem 1. What is the length of y?

ACT Math 1.2 Coordinate Geometry
252 Views

ACT Math: Coordinate Geometry Drill 1, Problem 2. Solve the inequality and determine which solution is shown on the number line.

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ACT Math 2.4 Coordinate Geometry 304 Views


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Description:

ACT Math: Coordinate Geometry Drill 2, Problem 4. What is the equation for the circle in the video?

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Transcript

00:02

Here's your shmoop du jour...

00:04

What is the equation for the circle shown?

00:06

And here are your potential answers...

00:10

If we don't know the formula for a circle, we're basically sunk.

00:13

That formula - just to refresh your brain -- is:

00:16

(x -- h)2 + (y -- k)2 = r2 where (h, k)

00:22

is the center of the circle and r is the radius.

00:25

We just have to figure out the h, k and r. One thing in our court that should make it

00:30

easier is that the circle's middle is the origin, or point 0, 0.

00:34

We can just plug those in for starters, which gives us the equation:

00:38

Quantity x minus 0 squared plus quantity y minus 0 squared equals r squared.

00:44

Get rid of the 0's and we have x squared plus y squared equals.....

00:49

Well, we know r already because the problem gives us 4 and negative 4 here and here...

00:55

so this distance is the radius

00:58

The circle's equation has to equal r squared... which is 16.

01:02

So the answer is D.

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