# ACT Math 2.4 Coordinate Geometry

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

ACT Math | Coordinate Geometry |

ACT Mathematics | Coordinate Geometry |

Algebra | Coordinates and Graphs |

Coordinate Geometry | Graphs of points, lines, polynomials, and circles The relations between equations and graphs |

Foreign Language | Arabic Subtitled Chinese Subtitled Korean Subtitled Spanish Subtitled |

Geometry | Coordinate Geometry |

Language | English Language |

Product Type | ACT Math |

### Transcript

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

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

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

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

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

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

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

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

so this distance is the radius

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

So the answer is D.