Thursday, January 16, 2014

College Algebra, Chapter 8, Review Exercises, Section Review Exercises, Problem 14

Determine the center, vertices, foci, eccentricity and lengths of the major and minor axes of the ellipse $\displaystyle \frac{(x - 2)^2}{25} + \frac{(y + 3)^2}{16} = 1$. Then sketch its graph.

The shifted ellipse has the form $\displaystyle \frac{(x - h)^2}{a^2} + \frac{(y - k)^2}{b^2} = 1$ with center at $(h,k)$. It is derived from the ellipse $\displaystyle \frac{x^2}{25} + \frac{y^2}{16} = 1$ with center at origin, by shifting it $2$ units to the right and $3$ units downward. The endpoints of the vertices of the unshifted ellipse are $(\pm 5, 0)$ and $(0, \pm 4)$. By applying transformations, the corresponding vertices of the shifted ellipse are..


$
\begin{equation}
\begin{aligned}

(5, 0) \to (5 + 2, 0 - 3) =& (7, -3)
\\
\\
(-5, 0) \to (-5 + 2, 0 - 3) =& (-3, -3)
\\
\\
(0, 4) \to (0 + 2, 4 - 3) =& (2, 1)
\\
\\
(0, -4) \to (0 + 2, -4 - 3) =& (2, -7)

\end{aligned}
\end{equation}
$



Now, the foci of the unshifted ellipse is determined by $c = \sqrt{a^2 - b^2} = \sqrt{25 - 16} = 3$. Then by applying transformations, the foci of the shifted ellipse are..


$
\begin{equation}
\begin{aligned}

(3, 0) \to (3 + 2, 0 - 3) =& (5, -3)
\\
\\
(-3, 0) \to (-3 + 2, 0 - 3) =& (-1, -3)

\end{aligned}
\end{equation}
$


To sum it up,

eccentricity $\displaystyle \frac{c}{a} \to \frac{3}{5}$

length of the major axis $2a \to 10$

length of the minor axis $\displaystyle 2b \to 8$

Therefore, the graph is

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