Question

How to calculate the Foci w/ the following equation:
in comment box

Answer 1

\[\frac{ x^2 }{ 81 } + \frac{ y^2 }{ 9 } = 1\]

Answer 2

@zepdrix @jim_thompson5910 @Hero

Answer 3

No one is going to help you if you don't show that you're trying. "The teacher used a different problem" is \(\sf \color{red}{NOT}\) an excuse for not attempting it.

Answer 4

see if that chart is useful

Answer 5

@Firejay5 You can ALWAYS try. Draw a picture. Make a sketch. Fill in what you know. Look at the lovely chart. Examine relationships.

How will you ever learn if you try only things you have already done? It's the ones you haven't seen that are important.

Answer 6

yea that's true @tkhunny . It frustrates me that people on this site think I don't even try, but how do they know that I don't have the answers on the other side

Answer 7

We are ghosts on the other side of your screen. You have to show us SOMETHING. We only think that you have done what you show us. We can't see into your head or into your room or into your class. You have to show us something.

Have you worked on any similar problems?
Is this sort of like anything you have seen?
Do you have a worked example?
It's an ellipse. Can we learn anything from parabolas or hyperbolas or circles?

That's the deal, really, if you show us nothing, we have nothing to go on. We can write you another text book, but is that really worth anyone's time? You already have a textbook.

Answer 8

use the equation
c^2 = a^2 - b^2

Answer 9

c^2 = 81 - 9
c^2 = 72
c = sqrt(72)

Answer 10

x^2/81+y^2/9=1,
By standard equation of eclipse
x^2/a^2+y^2/b^2=1,
Since a is always larger than b and c (a^2=b^2+c^2),
in this question the foci are on the X axis, and
\[c1=\sqrt{81-9}=6\sqrt{2}\],
and \[c2=-6\sqrt{2}\]
So foci are
\[(-6\sqrt{2},0),(6\sqrt{2},0)\]