Polar coordinates

Let the curve C be given by ${\displaystyle r=f(\theta ),}$ where the origin is taken to be O. Let A be the point (a, b). If ${\displaystyle K=(r\cos \theta ,\ r\sin \theta )}$ is a point on the curve the distance from K to A is

${\displaystyle d={\sqrt {(r\cos \theta -a)^{2}+(r\sin \theta -b)^{2}}}={\sqrt {(f(\theta )\cos \theta -a)^{2}+(f(\theta )\sin \theta -b)^{2}}}.}$

The points on the line OK have polar angle θ, and the points at distance d from K on this line are distance ${\displaystyle f(\theta )\pm d}$ from the origin. Therefore, the equation of the strophoid is given by

${\displaystyle r=f(\theta )\pm {\sqrt {(f(\theta )\cos \theta -a)^{2}+(f(\theta )\sin \theta -b)^{2}}}}$

Cartesian coordinates

Let C be given parametrically by (x(t), y(t)). Let A be the point (a, b) and let O be the point (p, q). Then, by a straightforward application of the polar formula, the strophoid is given parametrically by:

${\displaystyle u(t)=p+(x(t)-p)(1\pm n(t)),\ v(t)=q+(y(t)-q)(1\pm n(t)),}$

where

${\displaystyle n(t)={\sqrt {\frac {(x(t)-a)^{2}+(y(t)-b)^{2}}{(x(t)-p)^{2}+(y(t)-q)^{2}}}}.}$

An alternative polar formula

The complex nature of the formulas given above limits their usefulness in specific cases. There is an alternative form which is sometimes simpler to apply. This is particularly useful when C is a sectrix of Maclaurin with poles O and A.

Let O be the origin and A be the point (a, 0). Let K be a point on the curve, θ the angle between OK and the x-axis, and ${\displaystyle \vartheta }$ the angle between AK and the x-axis. Suppose ${\displaystyle \vartheta }$ can be given as a function θ, say ${\displaystyle \vartheta =l(\theta ).}$ Let ψ be the angle at K so ${\displaystyle \psi =\vartheta -\theta .}$ We can determine r in terms of l using the law of sines. Since

${\displaystyle {r \over \sin \vartheta }={a \over \sin \psi },\ r=a{\frac {\sin \vartheta }{\sin \psi }}=a{\frac {\sin l(\theta )}{\sin(l(\theta )-\theta )}}.}$

Let P₁ and P₂ be the points on OK that are distance AK from K, numbering so that ${\displaystyle \psi =\angle P_{1}KA}$ and ${\displaystyle \pi -\psi =\angle AKP_{2}.}$ △P₁KA is isosceles with vertex angle ψ, so the remaining angles, ${\displaystyle \angle AP_{1}K}$ and ${\displaystyle \angle KAP_{1},}$ are ${\displaystyle {\tfrac {\pi -\psi }{2}}.}$ The angle between AP₁ and the x-axis is then

${\displaystyle l_{1}(\theta )=\vartheta +\angle KAP_{1}=\vartheta +(\pi -\psi )/2=\vartheta +(\pi -\vartheta +\theta )/2=(\vartheta +\theta +\pi )/2.}$

By a similar argument, or simply using the fact that AP₁ and AP₂ are at right angles, the angle between AP₂ and the x-axis is then

${\displaystyle l_{2}(\theta )=(\vartheta +\theta )/2.}$

The polar equation for the strophoid can now be derived from l₁ and l₂ from the formula above:

${\displaystyle {\begin{aligned}&r_{1}=a{\frac {\sin l_{1}(\theta )}{\sin(l_{1}(\theta )-\theta )}}=a{\frac {\sin((l(\theta )+\theta +\pi )/2)}{\sin((l(\theta )+\theta +\pi )/2-\theta )}}=a{\frac {\cos((l(\theta )+\theta )/2)}{\cos((l(\theta )-\theta )/2)}}\\&r_{2}=a{\frac {\sin l_{2}(\theta )}{\sin(l_{2}(\theta )-\theta )}}=a{\frac {\sin((l(\theta )+\theta )/2)}{\sin((l(\theta )+\theta )/2-\theta )}}=a{\frac {\sin((l(\theta )+\theta )/2)}{\sin((l(\theta )-\theta )/2)}}\end{aligned}}}$

C is a sectrix of Maclaurin with poles O and A when l is of the form ${\displaystyle q\theta +\theta _{0},}$ in that case l₁ and l₂ will have the same form so the strophoid is either another sectrix of Maclaurin or a pair of such curves. In this case there is also a simple polar equation for the polar equation if the origin is shifted to the right by a.

Oblique strophoids

Let C be a line through A. Then, in the notation used above, ${\displaystyle l(\theta )=\alpha }$ where α is a constant. Then ${\displaystyle l_{1}(\theta )=(\theta +\alpha +\pi )/2}$ and ${\displaystyle l_{2}(\theta )=(\theta +\alpha )/2.}$ The polar equations of the resulting strophoid, called an oblique strphoid, with the origin at O are then

${\displaystyle r=a{\frac {\cos((\alpha +\theta )/2)}{\cos((\alpha -\theta )/2)}}}$

and

${\displaystyle r=a{\frac {\sin((\alpha +\theta )/2)}{\sin((\alpha -\theta )/2)}}.}$

It's easy to check that these equations describe the same curve.

Moving the origin to A (again, see Sectrix of Maclaurin) and replacing −a with a produces

${\displaystyle r=a{\frac {\sin(2\theta -\alpha )}{\sin(\theta -\alpha )}},}$

and rotating by ${\displaystyle \alpha }$ in turn produces

${\displaystyle r=a{\frac {\sin(2\theta +\alpha )}{\sin(\theta )}}.}$

In rectangular coordinates, with a change of constant parameters, this is

${\displaystyle y(x^{2}+y^{2})=b(x^{2}-y^{2})+2cxy.}$

This is a cubic curve and, by the expression in polar coordinates it is rational. It has a crunode at (0, 0) and the line y = b is an asymptote.

The right strophoid

A right strophoid

Putting ${\displaystyle \alpha =\pi /2}$ in

${\displaystyle r=a{\frac {\sin(2\theta -\alpha )}{\sin(\theta -\alpha )}}}$

gives

${\displaystyle r=a{\frac {\cos 2\theta }{\cos \theta }}=a(2\cos \theta -\sec \theta ).}$

This is called the right strophoid and corresponds to the case where C is the y-axis, A is the origin, and O is the point (a, 0).

The Cartesian equation is

${\displaystyle y^{2}=x^{2}(a-x)/(a+x).}$

The curve resembles the Folium of Descartes^[1] and the line x = –a is an asymptote to two branches. The curve has two more asymptotes, in the plane with complex coordinates, given by

${\displaystyle x\pm iy=-a.}$

Circles

Let C be a circle through O and A, where O is the origin and A is the point (a, 0). Then, in the notation used above, ${\displaystyle l(\theta )=\alpha +\theta }$ where ${\displaystyle \alpha }$ is a constant. Then ${\displaystyle l_{1}(\theta )=\theta +(\alpha +\pi )/2}$ and ${\displaystyle l_{2}(\theta )=\theta +\alpha /2.}$ The polar equations of the resulting strophoid, called an oblique strophoid, with the origin at O are then

${\displaystyle r=a{\frac {\cos(\theta +\alpha /2)}{\cos(\alpha /2)}}}$

and

${\displaystyle r=a{\frac {\sin(\theta +\alpha /2)}{\sin(\alpha /2)}}.}$

These are the equations of the two circles which also pass through O and A and form angles of ${\displaystyle \pi /4}$ with C at these points.