Relating X and Y

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• November 18th 2008, 08:42 AM
gearshifter
Relating X and Y
Eliminate t to give an equation that relates x and y:
x=cos(t)http://nernst.chem.sfu.ca/adm/jsMath...120/char3B.pngy=sin2(t)−3

y= ??
• November 18th 2008, 10:01 AM
Aryth
Remember the identity:

$\sin^2{t} + \cos^2{t} = 1$

So:

$sin^2{t} = 1 - \cos^2{t}$

$y = 1 - \cos^2{t} - 3$

$y = -\cos^2{t} - 2$

Now we look at x:

$x = \cos{t}$

Square both sides:

$x^2 = \cos^2{t}$

Multiply both sides by -1:

$-x^2 = -\cos^2{t}$

Now, subtract both sides by 2:

$-x^2 - 2 = -\cos^2{t} - 2$

We know that:

$y = -\cos^2{t} - 2$

So, we get:

$y = -x^2 - 2$

And there you go.
• November 18th 2008, 10:16 AM
earboth
Quote:

Originally Posted by gearshifter
Eliminate t to give an equation that relates x and y:
x=cos(t)http://nernst.chem.sfu.ca/adm/jsMath...120/char3B.pngy=sin2(t)−3

y= ??

Here is a slightly different attempt:

$x = \cos(t)~\implies~x^2=\cos^2(t)$

$y = \sin^2(t)-3~\implies~y+3=\sin^2(t)$

Add columnwise:

$y+3+x^2=\sin^2(t)+\cos^2(t)=1~\implies~\boxed{y=-x^2-2}$
• November 18th 2008, 07:55 PM
gearshifter
wow thanks. I was stuck on that for sometime because i didnt know how to relate it!