College Algebra (10th Edition)

Published by Pearson
ISBN 10: 0321979478
ISBN 13: 978-0-32197-947-6

Chapter 7 - Section 7.2 - The Parabola - 7.2 Assess Your Understanding - Page 515: 45

Answer

vertex: $\quad (3,-1)$ focus: $\displaystyle \quad (3,-\frac{5}{4})$ directrix: $\displaystyle \quad y=-\frac{3}{4}$

Work Step by Step

Comparing the form of the equation with table 2, $\quad \quad \quad (x-3)^{2}=-(y+1)\quad $has the form $\begin{array}{|c|c|c|c|} \hline {focus}&{directrix}&{equation}& ... opens\\ \hline {(h,k-a)}&{y=k+a}&{(x-h)^{2}=-4a(y-k)}& down \\ \hline \end{array}$ $(x-3)^{2}=-4\displaystyle \cdot(\frac{1}{4})(y+1)\quad \Rightarrow\quad h=3, k=-1, a=\displaystyle \frac{1}{4}$ vertex: $\quad (h,k)=(3,-1)$ focus: $\displaystyle \quad (h,k-a)=(3,-1-\frac{1}{4})=(3,-\frac{5}{4})$ directrix: $\displaystyle \quad y=k+a\quad \Rightarrow\quad y=-\frac{3}{4}$ To graph, find the endpoints of the latus rectum (line segment parallel to the directrix, containing the focus) by setting $y=-\displaystyle \frac{5}{4}$ $(x-3)^{2}=-(-\displaystyle \frac{5}{4}+1)$ $(x-3)^{2}=\displaystyle \frac{1}{4}$ $x-3=\displaystyle \pm\frac{1}{2}$ $x=3\displaystyle \pm\frac{1}{2}$ Endpoints of the latus rectum: $(2.5,-1.25), (3.5,-1.25)$
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