# Find the differential equation

• MatinSAR
In summary, the conversation discusses finding the equation and differential equation of a family of straight lines that are tangent to a given circle. The main goal is to rewrite the equation in terms of polar coordinates and then differentiate it to find the differential equation. The process involves finding the dot product between the radius vector and a perpendicular vector, and then solving for a constant, which represents the angle on the circle. The conversation also includes a discussion about the equation of a line in polar coordinates and how to differentiate it.
MatinSAR
Homework Statement
Find the differential equation of a family of straight lines that are tangent to the circle x^2+y^2=c^2?
Relevant Equations
Circle equation , mm'=-1,y-y1=m(x-x1)
Hi ... I have written the equation of family of straight lines which are tangent to the circle as :
y=(-m/n)x+(m^2/n)+n
line intersects circle at : (m,n)

But I can't understand how to find differential equation of this ...

I will be appreciated if anyone has extra time to give me a little guidance on this question.

I think the generic goal here is to rewrite the equation as lhs is a function of the coordinates, and rhs is a function of only the parameter that you pick to set which curve you have, and then you differentiate. Then integrating both sides gives you lhs = arbitrary number, and the choice of number sets your parameter (by inverting the function on the rhs)

You have two parameters (m and n) so you're in trouble to start. Your real parameter that picks your curve is the angle on the circle.

Does your differential equation have to be in Cartesian coordinates? I think it's a lot more natural in polar coordinates since you're trying to isolate an angle.

Step one is to write down the equation of a line in polar coordinates
Remember the tangent line to the circle is the set of points which in vector form are radius vector + something perpendicular to the radius vector. This means it's the set of points which, when you take the dot product with the radius vector of your set angle, gives you ##c^2##.

This actually gives you a really nice and simple representation of an arbitrary line perpendicular to the circle at angle ##\alpha## (remember the dot product can be computed using the lengths of the vectors and the angle between them!) I'll stop here and give you a chance to figure out the equation and solve the problem from here, let me know

MatinSAR
I am not sure what the problem is asking for, the differential equation of any family of straight lines is $$\frac{dy}{dx}=c$$ where c an arbitrary constant. Unless the problems wants to identify c in terms of the coordinates x and y of a point of circle. Yes it is $$c=-\frac{x}{y}$$ where (x,y) a point on the circle (or ##-\frac{m}{n}## since you used (m,n) for the point on circle.

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MatinSAR
BTW, what you did to find the general equation of tangent line is not wrong, but you could do it another way: Find the derivative ##\frac{dy}{dx}## of the equation of the circle (by implicit differentiation of the equation of the circle), then the equation of the tangent line that passes at point (x,y) of the circle is $$Y-y=\frac{dy}{dx}(X-x)$$.

I used capital X and Y for a point of the tangent line and small x and y for a point of the circle.

MatinSAR
Office_Shredder said:
Step one is to write down the equation of a line in polar coordinates
Thank you ... But can you share a link which describes how to write the equation of a line in polar coordinates?
Delta2 said:
I used capital X and Y for a point of the tangent line and small x and y for a point of the circle.
Thank you.

Delta2
@Office_Shredder the differential equation you proposing, will have differentiation with respect to which variable? If I understand well you mean differentiation with respect to some angle but what angle i didnt quite understand.

MatinSAR
Delta2 said:
@Office_Shredder the differential equation you proposing, will have differentiation with respect to which variable? If I understand well you mean differentiation with respect to some angle but what angle i didnt quite understand.

You should be able to describe an arbitrary line at ##f(r,\theta)= C## for some constant ##C## that only depends on your initial parameter (th angle that your point on the circle was at ). Then you can use implicit differentiation, just take the derivative with respect to either r or ##\theta## to be honest (though I think ##\theta## is more natural).

If you really need this to be in x and y terms you can turn ##r## and ##\theta## back into ##x## and ##y## and then differentiate with respect to ##x##.

MatinSAR
MatinSAR said:
Thank you ... But can you share a link which describes how to write the equation of a line in polar coordinates?

Thank you.
Do you know what I mean when I say 'the set of points on the tangent line are the set of points for which the dot product with ##(m,n)## is ##c^2##?

If so I can hit you with a linear algebra derivation, if not I'll have to think about the right way to present it.

Edit: an elementary construction. Suppose in polar coordinates the point on the circle is at ##(c,\alpha)## - ##c## is the distance from the origin and ##\alpha## is the angle from the x axis. Now pick a random angle ##\theta## and consider the choice of ##r## such that ##(r,\theta)## is on the line. You now have a right triangle - the radius line segment between the origin and ##(c,\alpha)##, the line segment between the origin and ##(r,\theta)## and the line segment on the tangent line between ##(c,\alpha)## and ##(r,\theta)##. Write down a trigonometric equation that this triangle satisfies. (What is the angle at the origin?)

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MatinSAR

## 1. What is a differential equation?

A differential equation is a mathematical equation that relates a function to its derivatives. It describes the relationship between the rate of change of a variable and the variables themselves.

## 2. Why do we need to find the differential equation?

Finding the differential equation allows us to model and understand the behavior of a system or process. It is used in many fields of science and engineering to make predictions and solve problems.

## 3. How do you find the differential equation of a system?

To find the differential equation of a system, you need to identify the variables and their rates of change, and then use mathematical operations to express their relationship. This can be done using physical laws, empirical data, or other mathematical models.

## 4. What are the different types of differential equations?

There are several types of differential equations, including ordinary differential equations (ODEs), partial differential equations (PDEs), and stochastic differential equations (SDEs). ODEs involve only one independent variable, while PDEs involve multiple independent variables. SDEs also involve random processes.

## 5. What are some applications of differential equations?

Differential equations are used in many areas of science and engineering, such as physics, chemistry, biology, economics, and engineering. They can be used to model population growth, chemical reactions, fluid flow, and many other processes. They are also used in fields like control theory and signal processing to design systems and predict their behavior.

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