Given the ellipse with equation 9x2 + 25y2 = 225, find the eccentricity and foci.
We know that equation of an ellipse is
$=\frac{\mathrm{x}^{2}}{\mathrm{a}^{2}}+\frac{\mathrm{y}^{2}}{\mathrm{~b}^{2}}=1$
Also we have,
Length of latus rectum $=\frac{2 b^{2}}{a}$
Length of minor axis $=2 b$
$9 x^{2}+25 y^{2}=225$
Dividing the above equation by 225 ,
$\frac{x^{2}}{25}+\frac{y^{2}}{9}=1$
The above equation can be written as
$\frac{x^{2}}{5^{2}}+\frac{y^{2}}{3^{2}}=1$
From the above equation we have,
$a=5, b=3$
$b^{2}=a^{2}\left(1-e^{2}\right)$
$3^{2}=5^{2}\left(1-e^{2}\right)$
On rearranging we get
$\frac{9}{25}=1-\mathrm{e}^{2}$
$e^{2}=1-\frac{9}{25}$
Taking LCM and simplifying we get
$e^{2}=\frac{16}{25}$
$e=\frac{4}{5}$
$3^{2}=5^{2}\left(1-e^{2}\right)$
On rearranging we get
$\frac{9}{25}=1-\mathrm{e}^{2}$
$\mathrm{e}^{2}=1-\frac{9}{25}$
Taking LCM and simplifving we get
$e^{2}=\frac{16}{25}$
$e=\frac{4}{5}$
We know foci $=(\pm a e, 0)$
$=\left(\pm 5 \times \frac{4}{5}, 0\right)=(\pm 4,0)$
Hence, the eccentricity is $4 / 5$ and foci is $(\pm 4,0)$.