Chapter 10.3, Problem 5GCE

To determine

Guess the behavior of the graph using given condition.

Explanation of Solution

Given:

The given equation is $\frac{{\left(x-3\right)}^2}{18}+\frac{{\left(y+2\right)}^2}{32}=1$ and the given condition is that the value of $32$ is replaced by $18$ .

Calculation:

The general equation of the ellipse is $\frac{{\left(x-h\right)}^2}{a^2}+\frac{{\left(y-k\right)}^2}{b^2}=1$ or $\frac{{\left(y-k\right)}^2}{a^2}+\frac{{\left(x-h\right)}^2}{b^2}=1$ .

If $a^2$ is the denominator of the $x$ term, the major axis is parallel to the x-axis. If $a^2$ is the denominator of the $y$ term, the major axis is parallel to the y-axis.

Original equation is

  $\frac{{\left(x-3\right)}^2}{18}+\frac{{\left(y+2\right)}^2}{32}=1$

In the equation, $a^2$ is the denominator of the $y$ term.

  $a^2=32$ , $b^2=18$ and $c^2=a^2-b^2=\sqrt{{32}^2-{18}^2}=10\sqrt{7}$

Center $\to \left(h,k\right)=\left(3,-2\right)$

Foci $\to \left(h,k\pm c\right)=\left(3,-2\pm 10\sqrt{7}\right)$

Major axis vertices $\to \left(h,k\pm a\right)=\left(3,-2\pm 4\sqrt{2}\right)$

Minor axis vertices $\to \left(h\pm b,k\right)=\left(3\pm 3\sqrt{2},-2\right)$

Replace the $32$ by $18$ .

  $\frac{{\left(x-3\right)}^2}{18}+\frac{{\left(y+2\right)}^2}{18}=1$

New equation is $\frac{{\left(x-3\right)}^2}{18}+\frac{{\left(y+2\right)}^2}{18}=1$ .

Here, only change in the value of $\text{'}a\text{'}$ .

In this equation $a^2=b^2=18$ , it is the form of a circle ${\left(x-h\right)}^2+{\left(y-k\right)}^2=r^2$ .

  ${\left(x-3\right)}^2+{\left(y+2\right)}^2={\left(3\sqrt{2}\right)}^2$

So, the new equation represents a circle with radius $3\sqrt{2}$ .

Graph of the functions are given below.

  

In the graph, blue curve shows the original equation and the red curve shows circle with radius $3\sqrt{2}$ .