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## Algebra (all content)

### Unit 17: Lesson 5

Center and radii of an ellipse# Ellipse graph from standard equation

Given the standard equation of an ellipse, Sal finds the graph of the ellipse.

## Want to join the conversation?

- hey, just asking, but in my math curriculum we had something like this, 3[x-1] squared, plus 3[y plus 3] squared. anybody understand?(1 vote)
- You have 3(x-1)²+3(y+1)², and I assume this was set equal to a constant. I'll name the constant "3r²".

So 3(x-1)²+3(y+1)²=3r²

Divide by 3 and get (x-1)²+(y+1)²=r²

So this is the equation of a circle centered at (1, -1) with a radius of r.(10 votes)

- hey guys an ellipse has a as length of 'radius' parallel to the horizontal axis

and b the 'radius' parallel to the vertical axis, while a hyperbola has a as whichever a or b is larger, right? it's still x^2/a^2 except a is always larger than b, right?(3 votes)- For an ellipse, either the
**a**or the**b**can be greater. When they are equal it forms a circle.(1 vote)

- Can you go over a question that is more complicated? I get the basics but I need more help on the intermediate stuff. Thanks!(3 votes)
- Hello, if you would like to learn more about conic sections in higher detail, check out this book.

https://artofproblemsolving.com/store/item/intermediate-algebra(1 vote)

- An ellipse has an infinite number of radii like circles, or just two ?(1 vote)
- There is no segment called the radius in an ellipse. The definition of a circle is the set of all points equidistant from a fixed point called the center (this distance is the radius). The definition of an ellipse is the set of all points such that the
*sum*of the distances from two fixed points called the foci (plural of focus) is constant.

Imagine having two nails in a board. If you tie a string at each end to the nails and pull the string taught with your finger, you can trace an ellipse with your finger by moving it around while keeping the string taught (though you'd have to lift up the string over the nail when crossing one of the vertices). The length of the string obviously doesn't change. That length is the defining characteristic of the ellipse (along with the foci - where you place the two nails in the board).(2 votes)

- write an equation for an ellipse if the endpoints of the major axis are at (1,6) and (1,-6) and the endpoints of the minor axis are at (5,0) and (-3,0)(1 vote)
- How would you write an equation for ellipses if it is tilted?(1 vote)
- To do that, you probably should do the trig thing to get the horizontal and vertical radius.(1 vote)

## Video transcript

- [Voiceover] We're asked
which ellipse is represented by the equation x minus
four squared over 16 plus y minus one squared
over 49 is equal to one. And we're given a bunch of choices here. We're given four choices here. So let's just think about
what's going on here. So the center of the ellipse
is going to be four comma one. How do I know that? Well the equation of the
ellipse is going to be x minus the x-coordinate for
the center squared over here, over the horizontal axis is
horizontal radius squared plus y minus the y-coordinate
of the center squared over the vertical radius squared. So the center is going
to be four comma one. So the center here is not four comma one. The center over here
is not four comma one. Not four comma one. The only choice that has
a center at four comma one is this one over here. So we already know this is the choice without even looking at the horizontal and the vertical radius. But we can verify that this works out 'cause a horizontal
radius right over here, notice it goes this orange line, which can represent the horizontal radius. It has a length of four and so
the horizontal radius is four and so we see indeed that 16 is the horizontal radius
squared, this is four squared. And if we look at the
vertical radius here, we see it has a length of seven. We're going from y equals
one to y equals eight, has a length of seven. And we see in that equation that this indeed is seven squared. So that was pretty straight forward.