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A conic with equation $$ a x^2 + b y^2 = c $$ has two focus points, where $a=4$, $b=24$ and $c=65$. One of those focus points has a positive x-coordinate. To two decimal places, what is the value of that positive x-coordinate?

I got the answer as 14.83, seems a bit too big is that right? The above answer is according to this in my textbook enter image description here

enter image description here

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title:"Just convert this to radians?" the question seems to be asking for a coordinate, not an angle... –  Navin Oct 3 '12 at 4:38
    
@Navin Tks for pointing that out my old question was in my browsers cache, changed the title –  JackyBoi Oct 3 '12 at 4:43
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1 Answer

up vote 3 down vote accepted

Put the ellipse in standard form. Start from $4x^2+24y^2=65$. Divide through by $65$. We get $\frac{4}{65}x^2+\frac{24}{65}y^2=1$.

This is $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$, where $a^2=\frac{65}{4}$ and $b^2=\frac{65}{24}$. Finally, you want $\sqrt{a^2-b^2}$.

Your intuition is right: this is substantially smaller than your answer. To two decimal places, I get $3.68$.

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This is an elipse in standard position so the eccentricity is squareroot(1- (a^2/b^2). Correct? –  JackyBoi Oct 3 '12 at 4:57
    
Also take note the question is asking for focal point –  JackyBoi Oct 3 '12 at 4:57
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I found the $x$-coordinate of the focal point with positive $x$-coordinate, as the question asks. The eccentricity is not $a^2/b^2$, it is $\frac{\sqrt{a^2-b^2}}{a}$. If your book/notes do not have the relevant formulas, I am sure you can find them in the wikipedia article on the ellipse, there must be one! –  André Nicolas Oct 3 '12 at 5:00
    
andre please see my textbook picture, in the original post. If I am wrong please let me know. You and I agree to certain extent but the eccentricity I am not sure that is the formula that you want me to use –  JackyBoi Oct 3 '12 at 5:06
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That is equivalent to what I wrote. But I guess if you follow the book, you will first find $e$, which is $\sqrt{1-\frac{b^2}{a^2}}$ (mine was different but equivalent) and then you will multiply by $a$. You will get $\sqrt{a^2-b^2}$. Anyway, same answer. Eccentricity is about $0.91287$. –  André Nicolas Oct 3 '12 at 5:16
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