# I need some major help reviewing please

• Nov 29th 2011, 05:34 PM
blackflag10
I need some major help reviewing please
My calculus teacher gave us a review sheet today that has several problems on it that are like what is going to be on our test that we are having Thursday. The problem is that he will be out of town until Thursday morning, so I can't go and ask him for help. I will post two of the problems that I am having issues with here. Please help me!!

1) Consider the polar equation r=24/5+sinx
a)Transform the polar equation into rectangular coordinates.
b) Find the foci of the ellipse formed

2) a) If A is the point (0,0,1) find a point B so that the vector AB has length 6 and is parallel to <-1,4,1>

b) find an equation of the sphere for which the line segment from A to C (4,-6,9) is a diameter

• Nov 29th 2011, 06:03 PM
Prove It
Re: I need some major help reviewing please
Quote:

Originally Posted by blackflag10
My calculus teacher gave us a review sheet today that has several problems on it that are like what is going to be on our test that we are having Thursday. The problem is that he will be out of town until Thursday morning, so I can't go and ask him for help. I will post two of the problems that I am having issues with here. Please help me!!

1) Consider the polar equation r=24/5+sinx
a)Transform the polar equation into rectangular coordinates.
b) Find the foci of the ellipse formed

2) a) If A is the point (0,0,1) find a point B so that the vector AB has length 6 and is parallel to <-1,4,1>

b) find an equation of the sphere for which the line segment from A to C (4,-6,9) is a diameter

Is this \displaystyle \displaystyle \begin{align*} r = \frac{24}{5} + \sin{x} \end{align*} or \displaystyle \displaystyle \begin{align*} \frac{24}{5\sin{x}} \end{align*}?
• Nov 29th 2011, 06:07 PM
blackflag10
Re: I need some major help reviewing please
It is r= 24/(5+sinx)
• Nov 29th 2011, 06:20 PM
Prove It
Re: I need some major help reviewing please
Quote:

Originally Posted by blackflag10
It is r= 24/(5+sinx)

You should know that \displaystyle \displaystyle \begin{align*} x = r\cos{\theta}, y = r\sin{\theta} \end{align*} and \displaystyle \displaystyle \begin{align*} x^2 + y^2 = r^2 \end{align*}

Therefore

\displaystyle \displaystyle \begin{align*} r &= \frac{24}{5 + \sin{x}} \\ r &= \frac{24}{5 + \frac{y}{r}} \\ r\left(5 + \frac{y}{r}\right) &= 24 \\ 5r + y &= 24 \\ 5\sqrt{x^2 + y^2} &= 24 - y \\ 25\left(x^2 + y^2\right) &= \left(24 - y\right)^2 \\ 25x^2 + 25y^2 &= y^2 - 48y + 576 \\ 25x^2 + 24y^2 + 48y &= 576 \\ 25x^2 + 24\left(y^2 + 2y\right) &= 576 \\ 25x^2 + 24\left(y^2 + 2y + 1^2 - 1^2\right) &= 576 \\ 25x^2 + 24\left[\left(y + 1\right)^2 - 1\right] &= 576 \\ 25x^2 + 24\left(y + 1\right)^2 - 24 &= 576 \\ 25x^2 + 24\left(y + 1\right)^2 &= 600 \\ \frac{x^2}{24} + \frac{\left(y + 1\right)^2}{25} &= 1 \end{align*}

You should be able to find the foci now.
• Nov 29th 2011, 07:32 PM
blackflag10
Re: I need some major help reviewing please
Quote:

Originally Posted by Prove It
You should know that \displaystyle \displaystyle \begin{align*} x = r\cos{\theta}, y = r\sin{\theta} \end{align*} and \displaystyle \displaystyle \begin{align*} x^2 + y^2 = r^2 \end{align*}

Therefore

\displaystyle \displaystyle \begin{align*} r &= \frac{24}{5 + \sin{x}} \\ r &= \frac{24}{5 + \frac{y}{r}} \\ r\left(5 + \frac{y}{r}\right) &= 24 \\ 5r + y &= 24 \\ 5\sqrt{x^2 + y^2} &= 24 - y \\ 25\left(x^2 + y^2\right) &= \left(24 - y\right)^2 \\ 25x^2 + 25y^2 &= y^2 - 48y + 576 \\ 25x^2 + 24y^2 + 48y &= 576 \\ 25x^2 + 24\left(y^2 + 2y\right) &= 576 \\ 25x^2 + 24\left(y^2 + 2y + 1^2 - 1^2\right) &= 576 \\ 25x^2 + 24\left[\left(y + 1\right)^2 - 1\right] &= 576 \\ 25x^2 + 24\left(y + 1\right)^2 - 24 &= 576 \\ 25x^2 + 24\left(y + 1\right)^2 &= 600 \\ \frac{x^2}{24} + \frac{\left(y + 1\right)^2}{25} &= 1 \end{align*}

You should be able to find the foci now.

Thank you so much!!!

If someone could help me out with the second question, I would be very grateful!
• Nov 29th 2011, 08:03 PM
Prove It
Re: I need some major help reviewing please
Quote:

Originally Posted by blackflag10
My calculus teacher gave us a review sheet today that has several problems on it that are like what is going to be on our test that we are having Thursday. The problem is that he will be out of town until Thursday morning, so I can't go and ask him for help. I will post two of the problems that I am having issues with here. Please help me!!

1) Consider the polar equation r=24/5+sinx
a)Transform the polar equation into rectangular coordinates.
b) Find the foci of the ellipse formed

2) a) If A is the point (0,0,1) find a point B so that the vector AB has length 6 and is parallel to <-1,4,1>

b) find an equation of the sphere for which the line segment from A to C (4,-6,9) is a diameter