# Parametric equations?

• December 16th 2013, 12:00 PM
orange
Parametric equations?
Find the area of the graph enclosed by the parametric equations,

y = 6 cos(t) sin(t)

x= 6 cos^2(t)

Thus far I did some algebra and came up with 6x^2+6y^2= 36y, but am stuck after that.

Thank you!
• December 16th 2013, 12:10 PM
Plato
Re: Parametric equations?
Quote:

Originally Posted by orange
Find the area of the graph enclosed by the parametric equations,
y = 6 cos(t) sin(t)
x= 6 cos^2(t)
Thus far I did some algebra and came up with 6x^2+6y^2= 36y, !

Can you show $6x^2+6y^2=36y$ is the same as $x^2+(y-3)^2=9~?$

Now I did not check your algebra.
• December 16th 2013, 12:44 PM
romsek
Re: Parametric equations?
Quote:

Originally Posted by orange
Find the area of the graph enclosed by the parametric equations,

y = 6 cos(t) sin(t)

x= 6 cos^2(t)

Thus far I did some algebra and came up with 6x^2+6y^2= 36y, but am stuck after that.

Thank you!

Did you plot the curve? The answer is pretty obvious if you do.

To do it formally though

$y^2(t)=36cos^2(t)sin^2(t)=6x(t)(1-\frac{x(t)}{6})$

dropping the t for convenience

$y^2=6x(1-\frac{x}{6})=6x-x^2$

$y=\pm\sqrt{6x-x^2}$

so find the area under $y(x)=\sqrt{6x-x^2}$ and double it to include the area under the $-\sqrt{6x-x^2}$

This will all be much clearer if you plot these functions.

As another trickier way of doing this

$y^2(t)=36cos^2(t)sin^2(t)=36cos^2(t)(1-cos^2(t))$

$y^2=6x(1-\frac{x}{6})=6x-x^2=-(x^2-6x+9-9)=-(x-3)^2+9$

$(x-3)^2+y^2=9$

This is immediately recognized as a circle of radius 3 centered at (3,0).

It's area is $\pi(3)^2=9\pi$
• December 16th 2013, 01:06 PM
Soroban
Re: Parametric equations?
Hello, orange!

Quote:

Find the area of the area enclosed by the parametric equations: . $\begin{Bmatrix}x &=& 6\cos^2\!t \\ y &=&6\cos t\sin t \end{Bmatrix}$

Square both equations: . $\begin{Bmatrix}x^2 &=& 36\cos^4\!t \\ y^2 &=& 36\cos^2\!t\sin^2\!t \end{Bmatrix}$

Add: . . . . . $x^2+y^2 \;=\;36\cos^4\!t + 36\cos^2\!t\sin^2\!t$

. . . . . . . . . $x^2+y^2\;=\;36\cos^2\!t\underbrace{(\cos^2\!t + \sin^2\!t)}_{\text{This is 1}}$

. . . . . . . . . $x^2 + y^2\;=\;36\cos^2\!t$

. . . . . . . . . $x^2+y^2 \;=\;6(6\cos^2\!t)$

. . . . . . . . . $x^2 + y^2 \;=\;6x$

. . . . . $x^2 - 6x + y^2 \;=\;0$

. . $x^2 - 6x + 9 + y^2 \:=\:9$

. . . . $(x-3)^2 + y^2 \;=\;9$

This is a circle with radius 3.
• December 16th 2013, 04:18 PM
orange
Re: Parametric equations?
Thank you for all these wonderful explanations!