MHB Janet's question at Yahoo Answers involving the Witch of Agnesi

  • Thread starter Thread starter MarkFL
  • Start date Start date
AI Thread Summary
The discussion revolves around parameterizing the Witch of Agnesi curve derived from a geometric setup involving a circle and specific points in the xy-plane. The parameterization is established as W(t) = (2a tan t, 2a cos² t), with x_M and y_M defined in relation to point P on the circle. The Cartesian equation is derived by eliminating the parameter t, resulting in y = 8a³ / (x² + 4a²). This transformation showcases the relationship between the parameterization and the Cartesian form of the curve. The discussion emphasizes the mathematical derivation and properties of the Witch of Agnesi.
MarkFL
Gold Member
MHB
Messages
13,284
Reaction score
12
Here is the question:

Parameterize and cartesian equation problem?

If P is any point on the circle C in the xy-plane of radius a >0 and center (0,a),let the straight line through the origin and P intersect the line y = 2a at Q, and let the line through P parallel to the x-axis intersect the line through Q parallel to the y-axis at M. As P moves around C, M traces out a curve called the witch of Agnesi.
For this curve, prove that it can be parameterized as W (t) = (2a tan t, 2a cos2 t). Fi- nally, use this parameterization to find a cartesian equation for the curve by eliminating the variable t.

Here is the original question:

Parameterize and cartesian equation problem? - Yahoo! Answers

I have posted a link there to this topic so the OP can find my response.

edit: I see now that the question has been deleted. According to the guidelines there it is okay to post links to a site to offer more information on a question, so I can only speculate as to why it was deleted. (Headbang)
 
Last edited:
Mathematics news on Phys.org
Hello Janet,

Please refer to the following diagram:

View attachment 570

Let point M be at $(x_M,y_M)$, and point P be at $(x_P,y_P)$

As you can see, we may state:

$\displaystyle \tan(t)=\frac{x_M}{2a}\,\therefore\,x_M=2a\tan(t)$

We may also state:

$\displaystyle \tan(t)=\frac{x_P}{y_M}$

$\displaystyle y_M=x_P\cot(t)=a\sin(2t)\cot(t)=2a\cos^2(t)$

I made the observation that:

$\displaystyle x_P=a\sin(2t)$ from the requirements:

$\displaystyle x_P(0)=0,\,x_P\left(\frac{\pi}{4} \right)=a,\,x_P\left(\frac{\pi}{2} \right)=0$

And so we have the parametrization:

$\displaystyle M(t)=\langle 2a\tan(t),2a\cos^2(t) \rangle$

Now, to eliminate the parameter to obtain a Cartesian representation of the curve, we may write:

$\displaystyle t=\tan^{-1}\left(\frac{x}{2a} \right)$

Now substituting into the y-component, we find:

$\displaystyle y=2a\cos^2\left(\tan^{-1}\left(\frac{x}{2a} \right) \right)=\frac{8a^3}{x^2+4a^2}$
 

Attachments

  • witchofAgnesi.jpg
    witchofAgnesi.jpg
    7.1 KB · Views: 106
Last edited:
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Fermat's Last Theorem has long been one of the most famous mathematical problems, and is now one of the most famous theorems. It simply states that the equation $$ a^n+b^n=c^n $$ has no solutions with positive integers if ##n>2.## It was named after Pierre de Fermat (1607-1665). The problem itself stems from the book Arithmetica by Diophantus of Alexandria. It gained popularity because Fermat noted in his copy "Cubum autem in duos cubos, aut quadratoquadratum in duos quadratoquadratos, et...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Back
Top