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Ellipse equation review

Review your knowledge of ellipse equations and their features: center, radii, and foci.

What is the standard equation of an ellipse?

start fraction, left parenthesis, x, minus, h, right parenthesis, squared, divided by, a, squared, end fraction, plus, start fraction, left parenthesis, y, minus, k, right parenthesis, squared, divided by, b, squared, end fraction, equals, 1
This is the standard equation of the ellipse centered at left parenthesis, h, comma, k, right parenthesis, whose horizontal radius is a and vertical radius is b.
Want to learn more about ellipse equation? Check out this video.

Check your understanding

Problem 1
Which ellipse is represented by the equation start fraction, left parenthesis, x, minus, 4, right parenthesis, squared, divided by, 9, end fraction, plus, start fraction, left parenthesis, y, plus, 6, right parenthesis, squared, divided by, 4, end fraction, equals, 1 ?
Choose 1 answer:
Choose 1 answer:

Want to try more problems like this? Check out this exercise.

Want to join the conversation?

  • hopper cool style avatar for user Ralph Turchiano
    Just for the sake of formality, is it better to represent the denominator (radius) as a power such as 3^2 or just as the whole number i.e. 9.
    (14 votes)
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    • old spice man blue style avatar for user arora18204
      That would make sense, but in a question, an equation would hardly ever be presented like that. It would make more sense of the question actually requires you to find the square root. Having 3^2 as the denominator most certainly makes sense, but it just makes the question a whole lot easier.
      (13 votes)
  • blobby green style avatar for user Garima Soni
    Please explain me derivation of equation of ellipse.
    (6 votes)
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  • orange juice squid orange style avatar for user Matthew Johnson
    *Would the radius of an ellipse match the radius in the beginning of a parabola or hyperbola?* How could we calculate the area of an ellipse? Do they occur naturally in nature? Do they have any value in the real world other than mirrors and greeting cards and JS programming (https://www.khanacademy.org/computer-programming/spin-off-of-ellipse-demonstration/5350296801574912)? Can they be classified as circles? Or would circles be classified as an ellipse? How could the circumference of an ellipse be measured? Would you still use π? What about a cylinder with a base that is an ellipse instead of a circle? How would formulas change for manipulating circles vs. ellipse?
    (4 votes)
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  • starky tree style avatar for user Abi
    What if the center isn't the origin? Is the equation still equal to one?
    (3 votes)
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  • blobby green style avatar for user 😊
    what isProving standard equation of an ellipse??
    (3 votes)
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  • blobby green style avatar for user Fred Haynes
    A simple question that I have lost sight of during my reviews of Conics. Why is the standard equation of an ellipse equal to 1?
    (1 vote)
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    • leaf green style avatar for user kubleeka
      The standard equation of a circle is x²+y²=r², where r is the radius. An ellipse is a circle that's been distorted in the x- and/or y-directions, which we do by multiplying the variables by a constant. e.g. we stretch by a factor of 3 in the horizontal direction by replacing x with 3x.

      If we stretch the circle, the original radius of the circle becomes irrelevant. We could just as well have started with a smaller circle and stretched it more. So we normalize the equation, and get rid of the irrelevant 'r' variable, by just dividing both sides by the original r². The right-hand side is just 1 in every case, and all the information about the stretching is encoded in just the two coefficients.

      We could have chosen a different constant to make the right-hand side equal to, and it would have served the same purpose. But 1 is simplest to work with, so we use that.
      (1 vote)
  • blobby green style avatar for user Fred Haynes
    This is on a different subject. But what gives me the right to change (p-q) to (p+q) and what's it called?
    (1 vote)
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  • blobby green style avatar for user Peyton
    How do you change an ellipse equation written in general form to standard form. Ex: changing x^2+4y^2-2x+24y-63+0 to standard form
    (1 vote)
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  • blobby green style avatar for user kananelomatshwele
    How do I find the equation of the ellipse with centre (0,0) on the x-axis and passing through the point (-3,2*3^2/2) and (4,4/3*5^1/2)?
    (1 vote)
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    • leaf grey style avatar for user dashpointdash
      The ellipse is centered at (0,0) but the minor radius is uneven (-3,18?) and (4,4/3*sqrt(5)?).

      We know the ellipse equation to be
      x^2/a^2 + y^2/b^2 = 1, where a is the first, b the second radius.

      So - now, we have got 2 points that are satisfied by the equation.

      Equation
      I.:
      9/a^2 + 324/b^2 = 1

      II.:
      16/a^2 + (32/9)/b^2 = 1

      From here we solve by elimination LCM of 9 and 16 is I_.:
      144/a^2 + 16*324/b^2 = 16

      II_.:
      144/a^2 + 9*(32/9)/b^2 = 9

      I_ - II_.:
      16*324/b^2 + 9*(32/9)/b^2 = 7
      b^2 = 7/5216

      Insert the value in II. and a^2 is -0.006
      => there is no solution

      (in case I got the numbers wrong, just change them - but the procedure should be the same. Get equations and solve for radii - this all asumes math without tilted ellipses, which is a tad bit more complicated :))
      (1 vote)
  • starky tree style avatar for user Dakari
    Is there a specified equation a vertical ellipse and a horizontal ellipse or should you just use the standard form of an ellipse for both?
    (1 vote)
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