# find equation of ellipse????

• Apr 29th 2012, 05:27 PM
sluggerbroth
find equation of ellipse????
foci (0, +/- 4/3), contains (4/5,1)
• Apr 29th 2012, 05:55 PM
skeeter
Re: find equation of ellipse????
Quote:

Originally Posted by sluggerbroth
foci (0, +/- 4/3), contains (4/5,1)

centered at the origin (why?)

$\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1$

$c = \frac{4}{3}$

$b^2 = a^2 + c^2$

same drill ... sub in $x$ and $y$, solve for $a^2$ and $b^2$
• Apr 29th 2012, 06:02 PM
sluggerbroth
Re: find equation of ellipse????
algera gets really, really messy??? or am i doing something wrong?????
• Apr 29th 2012, 06:26 PM
skeeter
Re: find equation of ellipse????
yes, this one is messy ...

$\frac{(4/5)^2}{a^2} + \frac{1^2}{b^2} = 1$

$b^2 = a^2 + \frac{16}{9}$

$\frac{16}{25a^2} + \frac{1}{b^2} = 1$

$\frac{16}{25a^2} + \frac{1}{a^2 + \frac{16}{9}} = 1$

$\frac{16}{25a^2} + \frac{9}{9a^2 + 16} = 1$

$16(9a^2+16) + 9(25a^2} = 25a^2(9a^2+16)$

$144a^2 + 256 + 225a^2 = 225a^4 + 400a^2$

$0 = 225a^4 -31a^2 - 256$

$0 = (225a^2-256)(a^2 + 1)$

$a^2 = \frac{256}{225}$

sometimes you just have to grind it out