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## Algebra (all content)

### Course: Algebra (all content)>Unit 17

Lesson 5: Center and radii of an ellipse

# Ellipse equation review

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

## What is the standard equation of an ellipse?

$\frac{\left(x-h{\right)}^{2}}{{a}^{2}}+\frac{\left(y-k{\right)}^{2}}{{b}^{2}}=1$
This is the standard equation of the ellipse centered at $\left(h,k\right)$, whose horizontal radius is $a$ and vertical radius is $b$.

Problem 1
Which ellipse is represented by the equation $\frac{\left(x-4{\right)}^{2}}{9}+\frac{\left(y+6{\right)}^{2}}{4}=1$ ?

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

## Want to join the conversation?

• 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.
• 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.
• Please explain me derivation of equation of ellipse.
• *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?
• What if the center isn't the origin? Is the equation still equal to one?
• Regardless of where the ellipse is centered, the right hand side of the ellipse equation is always equal to 1.
• Can someone explain to me the derivation of the Ellipse Equation? Sal explained in the intro video how it's similar to the circle, with just the radius being unequal in an ellipse, thus "a" and "b" for the denominator. However, this is still unclear to me... If someone can provide a detailed but intuitive explanation, that would be very helpful.
• what isProving standard equation of an ellipse??
• 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)
• The algebraic rule that allows you to change (p-q) to (p+q) is called the "additive inverse property." This property states that the sum of a number and its additive inverse is always equal to zero.

In this case, the additive inverse of q is -q. Therefore, if you add -q to p-q, you get:

p - q + (-q) = p - (q + q) = p - 2q

However, if you instead add q to both sides of p-q, you get:

p - q + q = p

And thus, you can substitute (p+q) for (p-q) if it makes sense in the context of the problem you are trying to solve. It is important to note that this substitution is only valid if you are adding or subtracting the same value to both sides of an equation, and you should be careful to keep track of any changes you make to ensure that your answer is still correct.
• 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)
• 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 :))