# An ellipse question?

i added a file with the curve the is being asked to find

an ellipse is given. its formula is x^2 + 2*y^2=8 .

find the formula of the curve that is being created by the

the centers of the perpendicular lines to the X axes and the ellipse.

i tried to solve this question
by putting the Y^2 on the one side and on the other the rest
and devidind it by 2 .

2y^2=8-x^2
y^2=8-x^2/2
y=V(8-x^2/2)
(v=root simbol)

the curve that in the center always smaller in height by 2
so i devided the formula by 2 to find our curve

y=1/2*V(8-x^2/2)
in my book it gives a different answer

(x^2)/8 +y^2=1

plz help

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HallsofIvy
Homework Helper
What do you mean by "the centers of the perpendicular lines to the X axes and the ellipse"?

like it shows at the pictue
a curve wich composed from the middle points of this straight lines
parrallel to the Y axes

how can i solve this thing???

HallsofIvy
Homework Helper
$x^2+ 2y^2= 8$ is the same as $\frac{x^2}{8}+ \frac{y^2}{4}= 1$.
That's an ellipse with major semi-axis, along the x-axis, of length $\sqrt{8}= 2\sqrt{2}$ and minor semi-axis, along the y-axis, of length 2.

Dividing the y-coordinate of each point by 2 gives an ellipse with the same major semi-axis but minor semi-axis of length 1:
$\frac{x^2}{8}+ y^2=1$.

HallsofIvy
Homework Helper
i added a file with the curve the is being asked to find

an ellipse is given. its formula is x^2 + 2*y^2=8 .

find the formula of the curve that is being created by the

the centers of the perpendicular lines to the X axes and the ellipse.

i tried to solve this question
by putting the Y^2 on the one side and on the other the rest
and devidind it by 2 .

2y^2=8-x^2
y^2=8-x^2/2

Here's your error! Don't know why I didn't see this sooner. Dividing both sides of 2y2= 8- x2 by 2 gives y2= 4- x2!

y=V(8-x^2/2)
(v=root simbol)

the curve that in the center always smaller in height by 2
so i devided the formula by 2 to find our curve

y=1/2*V(8-x^2/2)
$y= 1/2 \sqrt{4- x^2}$

in my book it gives a different answer

(x^2)/8 +y^2=1

plz help

Square both sides of $2y= \sqrt{4- x^2}$ and you get
$4y^2= 4- x^2$ or $x^2+ 4y^2= 4$.

Divide through by 4:
[tex]\frac{x^2}{4}+ y^2= 1[/itex]

thank you very much

HallsofIvy