Diff Equation problem- give me a nudge, please

  • Thread starter Latecomer
  • Start date
In summary, this equation is illegal to solve because it does not have a simple solution that fits in explicit form.
  • #1
Latecomer
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Homework Statement



(dy/dx) - cos2(x-y) = 0

Homework Equations





The Attempt at a Solution



I'm unsure where to start this. This isn't a liner equation of form (dy/dt) + p(t)y = g(t) ; and it's not separable nor an exact equation.

Is it illegal to make this (dy/dx) - cos2(x) + cos2y = 0 ?


Thanks for your time.
 
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  • #2
Latecomer said:

Is it illegal to make this (dy/dx) - cos2(x) + cos2y = 0 ?
It is totally illegal!:rolleyes:

Hint: replace y-x with a new variable.

ehild
 
  • #3
Yeah, I knew that was illegal. I was just grasping at straws because I had no idea where to begin. And it's early...

Well, I have not worked with problems like this at all and I'm having trouble finding anything similar in my text, but this is what I have figured out so far:

(dy/dx) -cos2(x-y) = 0

(dy/dx) = cos2(x-y) make (x-y) = v

(dy/dx) = cos2(v)

v = x - y
y = x - v
y' = 1 - v'

from original : y' = cos2(x - y)
so: 1 - v' = cos2(v)
and : v' = 1 - cos2(v)

Am I heading in the right direction? Another nudge? Thanks again.
 
  • #4
It is all right so far. Is not it a separable de?

ehild
 
  • #5
Remember that 1-cos2(v) = sin2(v).
 
  • #6
Yes, I should have seen that.

(dv/dx) = 1-cos2(v)

dv/1-cos2(v) = dx

which is : csc^2(v) = dx

integrate:

-cot(v) = x + c

-cot (x - y) = x + c

Thank you for your help. I was just staring dumbly at it.

This implicit form should be a suitable answer for this question, yes? I was just told to solve the equation.
 
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  • #7
It should be suitable, but it's a simple process to get the answer in explicit form, if you want it.
 
  • #8
Hmm...

-cot (x-y) = x + c

cot (x-y) = -x +c

(x - y) = arccot (-x + c)

-y= arccot (-x + c) - x

y = x - arccot (-x + c) ?
 
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  • #9
Hint: The inverse of the cotangent function is arccotangent, not arctangent.
 
  • #10
hehe, yeah I saw that as soon as I submitted it and then edited it. You're fast :redface:
 
  • #11
Looks good to me. You can plug it in and test it if you want.
 

FAQ: Diff Equation problem- give me a nudge, please

What is a differential equation?

A differential equation is an equation that involves one or more derivatives of an unknown function. It relates the rate of change of the function to its current value and other variables.

Why are differential equations important?

Differential equations are important because they are used to describe many natural phenomena and physical processes. They are also essential in many fields, such as engineering, physics, economics, and biology, for modeling and analyzing complex systems.

What are some common methods for solving differential equations?

There are several methods for solving differential equations, including separation of variables, substitution, and using integrating factors. Other techniques include Laplace transforms, power series, and numerical methods such as Euler's method and Runge-Kutta methods.

How can I get started with solving a differential equation problem?

To solve a differential equation problem, you should first start by identifying the type of differential equation and determining the appropriate method to solve it. Then, you can begin by writing out the equation and any given initial conditions. From there, you can apply the chosen method to find a general solution and use the initial conditions to find a particular solution.

What are some common mistakes to avoid when solving differential equation problems?

Some common mistakes to avoid when solving differential equation problems include not properly identifying the type of differential equation, using incorrect methods, making mistakes in algebraic manipulations, and not checking the solution for accuracy. It is also important to be careful when dealing with initial conditions and to use proper notation when writing out the solution.

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