Saturday, September 1, 2012

College Algebra, Chapter 8, Review Exercises, Section Review Exercises, Problem 34

Identify the type of curve which is represented by the equation$\displaystyle 32 x^2 + 6 x = 9y^2$
Find the foci and vertices(if any), and sketch the graph

$
\begin{equation}
\begin{aligned}
x^2 + 6x - 9y^2 &= 0 && \text{Subtract } 9y^2\\
\\
x^2 + 6x + 9 - 9y^2 &= 9 && \text{Complete the square: Add } \left( \frac{6}{2} \right)^2 = 9\\
\\
(x + 3)^2 - 9y^2 &= 9 && \text{Perfect Square}\\
\\
\frac{(x+3)^2}{9} - y^2 &= 1 && \text{Divide by 9}
\end{aligned}
\end{equation}
$

The equation is hyperbola that has the form $\displaystyle \frac{(x-h)^2}{a^2} - \frac{(y - k)^2}{b^2} = 1$ with center at $(h,k)$ and horizontal transverse axis
because the denominator of the $x^2$ is positive. Thus gives $a^2 = 9$ and $b^2 = 1$, so $a = 3, b =1 $ and $c = \sqrt{a^2 + b^2} = \sqrt{9+1} = \sqrt{10}$. The
graph of the shifted ellipse is obtained by shifting the graph of $\displaystyle \frac{x^2}{9} - y^2 = 1$ three units to the left. Thus by applying transformation

$
\begin{equation}
\begin{aligned}
\text{center } & (h,k) && \rightarrow && (-3,0)\\
\\
\text{vertices} & (a,0)&& \rightarrow && (3,0) && \rightarrow && (3-3,0) && = && (0,0)\\
\\
& (-a,0)&& \rightarrow && (-3,0) && \rightarrow && (-3-3,0) && = && (-6,0)\\
\\
\text{foci } & (c,0)&& \rightarrow && (\sqrt{10},0) && \rightarrow && (\sqrt{10}-3,0) && = && (-3+\sqrt{10},6)\\
\\
& (-c,0)&& \rightarrow && (-\sqrt{10},0) && \rightarrow && (-\sqrt{10}-3,0) && = && (-3-\sqrt{10},0)
\end{aligned}
\end{equation}
$

Therefore, the graph is

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