# Thread: Conics.

1. ## Conics.

Hello!

I have like 3 problems:

a. Produce a conic system of inequalities representing the "doughnut-shape" created by an ellipse inside a circle.

b. Produce a conic system that has exactly two points of intersection and consists of a circle and ellipse with the same area.

c. True or False? Every system of two conic equations whose xy-term is nonexistent has at most 4points of intersection.

I can start with a circle... I don't know how to start the equation for ellipse and what does it look like iside the circle?

2. Hello, Anemori!

a. Produce a conic system of inequalities representing the "doughnut-shape"
created by an ellipse inside a circle.
Code:
                |
..* * *..
*:::::|:::::*
*:::::::|:::::::*
*::::::::|::::::::*
:::::::::**::::::::
*::::*    |b   *::::* c
- - *:-:* - - + - - *:-:* - - -
*::::*    |  a *::::*
:::::::***::::::::
*::::::::|::::::::*
*:::::::|:::::::*
*:::::|:::::*
* * *
|

The equation of the ellipse is: . $\frac{x^2}{a^2} + \frac{y^2}{b^2} \:=\:1$

The equation of the circle is: . $x^2 + y^2 \:=\:c^2,\,\text{ where }c \,\geq\,a,b$

System: . $\begin{Bmatrix} \dfrac{x^2}{a^2} + \dfrac{y^2}{b^2} & \geq & 1 \\ \\[-3mm]x^2 + y^2 & \leq & c^2 \end{Bmatrix}$

b. Produce a conic system that has exactly two points of intersection
and consists of a circle and ellipse with the same area.

The equation of an ellipse is: . $\frac{x^2}{a^2} + \frac{y^2}{b^2} \:=\:1$
. . Its area is: . $\pi ab$

The equation of a circle is: . $x^2 + y^2 \:=\:r^2$
. . Its area is: . $\pi r^2$

The areas are equal: . $\pi ab \:=\:\pi r^2 \quad\Rightarrow\quad ab \:=\:r^2$

Let: . $a = 4,\;b = 1,\;r = 2$

The ellipse is: . $\frac{x^2}{16} + \frac{y^2}{1} \;=\;1$
Code:
               1|
o o o
o       |       o
o          |          o
- o - - - - - + - - - - - o - -
o          |          o 4
o       |       o
o o o
|

The circle is: . $x^2 + y^2 \:=\:4$
Code:
               2|
◊ ◊ ◊
◊     |     ◊
◊      |      ◊
|
- - ◊ - - - + - - - ◊ - -
|        2
◊      |      ◊
◊     |     ◊
◊ ◊ ◊
|

Move the circle 2 units to the right.

Code:
                |       :
|     ◊ ◊ ◊
o o ♥     :     ◊
o       |◊      o      ◊
o          |       :  o
- - o - - - - - ◊ - - - + - o - ◊ - -
o          |       :  o
o       |◊      o      ◊
o o ♥     :
|     ◊ ◊ ◊

System: . $\begin{Bmatrix}\dfrac{x^2}{16} + \dfrac{y^2}{1} \:=\:1 \\ \\[-3mm] (x-2)^2 + y^2 \:=\:4 \end{Bmatrix}$