An Ellipse Or A Circle? - Parametric Equations
Date: 12/05/98 at 18:49:16 From: Jacob Subject: Parametric equations Hi Doctor Math, My friend and I are in a debate over a parametric equation of: x = 4(1-sin t), y = 4(1-cos t) over the interval 0 < t < 2pi. I believe it is elliptical in the first coordinate and he says it is a circle in the first coordinate with center (4,4). What does it look like? Also we want to figure the slopes at points 0, pi/4, pi/2, 3pi/2, and 2pi? If it is a circle the answers are obvious, but if it's not, what would they be? We appreciate any help you can give us. Thank you. Jacob Sands
Date: 12/05/98 at 19:30:09 From: Doctor Barrus Subject: Re: Parametric equations Hi, Jacob! It's pretty easy to recognize conic sections by their equations when the equations are written in terms of x and y. What you can try to do, then, is come up with an equation in terms of x and y - eliminate all t's. There are a number of ways we can do this. I'm going to use the fact that (sin(t))^2 + (cos(t))^2 = 1 Solving your equations for sin(t) and cos(t), we get x = 4(1-sin t) => sin t = -(x-4)/4 y = 4(1-cos t) => cos t = -(y-4)/4 Then (sin t)^2 + (cos t)^2 = 1 means that [-(x-4)/4]^2 + [-(y-4)/4]^2 = 1 (x-4)^2 (y-4)^2 ------- + ------- = 1 16 16 (x-4)^2 + (y-4)^2 = 16, or x^2 + y^2 - 8x - 8y + 16 = 0 Since t ranges from 0 to 2pi, we're going to cover all possible sines and cosines, so we don't really have to worry about t's interval. From either of the last two equations above you can recognize that the conic section is a circle. The top one fits the form (x-h)^2 + (y-k)^2 = r^2 and so tells you that the circle is centered at (h,k) = (4,4) and has a radius of r = 4. This would place the circle in the first quadrant. It sounds like you know how to find the derivative at the given points, so I'll let you figure that out. If it turned out to be an ellipse, though, what you could do is use implicit differentiation on the x-y equation you arrived at at the end to find an equation for dy/dx. Knowing t, you could then use the equations you began with to figure out the appropriate x and y to plug into the derivative equation. I hope this has helped. If you have further questions, feel free to write back. Good luck! - Doctor Barrus, The Math Forum http://mathforum.org/dr.math/
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