(x^2/81)-(y^2/4)=1
Find the Foci of the parabola

Question

(x^2/81)-(y^2/4)=1
Find the Foci of the parabola

directrix · Answer

Are you sure this is a parabola? Both variables are squared.

anonymous · Answer

Oops, I meant hyperbola!

anonymous · Answer

Hyperbola, exactly :P

$$\frac{x^2}{81}-\frac{y^2}{4}=1$$
=> a=√81 = 9 
and b= √4 = 2

For a hyperbola,

$$a^2 + b^2 = c^2$$

And it's focus is given by : (±c,0)

anonymous · Answer

What do you get for vertices? @Aditi_Singh

anonymous · Answer

Erm .. The vertices would be : (±a,0)

directrix · Answer

So, that is (-9,0) and (9,0) vertices.

And, (√85, 0) and (-√85,0) for foci.

anonymous · Answer

Yep!

directrix · Answer

@Aditi_Singh  Would you give your opinion on another hyperbola question?
Three of us are not in agreement about the foci of this hyperbola. I'll post the thread here in the event you are up for the challenge. :)

anonymous · Answer

Haha :D please :) :P

directrix · Answer

I found the question, multiple choice, too.

Find the coordinates of the foci of the hyperbola y^2/9-x^2/16=1.

A) (4, 0) and (-4, 0) 
B) (5, 0) and (-5, 0) 
C) (0, square root 7) and  (0, -square root 7)
D) (0, 5) and (0, -5)

anonymous · Answer

I would go with 'D' - (0,±5)

directrix · Answer

Alrighty, now for the big "reveal."

anonymous · Answer

because, here the foci is on y axis ..

anonymous · Answer

*Fingers crossed*

directrix · Answer

http://openstudy.com/users/maddss#/updates/514e1555e4b0ae0b658aed6b

directrix · Answer

We're together.

anonymous · Answer

Hi5! xD

directrix · Answer

@sakigirl Here's a conic section practice test with answers. http://www.education.com/study-help/article/pre-calculus-help-pre-calculus-12-conic/

anonymous · Answer

@Directrix Thanks!

directrix · Answer

@Aditi_Singh   I tried three times to tell the Asker but she ignored me and went with the others. 

That two of the options had 5s in the them was a BIG clue.

directrix · Answer

@sakigirl  You could post those questions one by one here, tell in the post that you are practicing for a test tomorrow, and practice that way.

anonymous · Answer

Awh! 
I just went through the whole question ..
And .. I .... I .. I'm speechless :P

directrix · Answer

My gosh. Out of desperation, I posted the graph. But, no, I was ignored.

anonymous · Answer

Hahah :P
Ignore them - as simple as that :P