Solving an Initial Value Problem with a Sinusoidal Differential Equation

Click For Summary
SUMMARY

The discussion focuses on solving the initial value problem defined by the differential equation \(\frac{dw}{d\theta}=\theta w^{2}\sin(\theta^{2})\) with the initial condition \(w(0)=1\). The proposed solution, \(w=\frac{2}{1+\cos(\theta^{2})}\), contains errors primarily due to mismanagement of the arbitrary constant during integration. Key corrections include isolating the function variable \(w(\theta)\) before determining the constant and recognizing that any value multiplied by an arbitrary constant remains an arbitrary constant. Following these steps will lead to the correct solution.

PREREQUISITES
  • Understanding of differential equations, specifically first-order nonlinear equations.
  • Familiarity with initial value problems and their solutions.
  • Knowledge of integration techniques, particularly u-substitution.
  • Basic trigonometric identities and their implications in calculus.
NEXT STEPS
  • Review the method of solving first-order nonlinear differential equations.
  • Study the process of determining arbitrary constants in initial value problems.
  • Learn about trigonometric identities and their application in calculus.
  • Practice integration techniques, focusing on u-substitution and rearranging equations.
USEFUL FOR

Students and educators in mathematics, particularly those studying differential equations, as well as anyone involved in solving initial value problems in calculus.

JamesGoh
Messages
140
Reaction score
0
For the following problem

\frac{dw}{d\theta}=\theta w^{2}sin(\theta^{2}), w(0)=1

I am not able to obtain the solution

w=\frac{2}{1+cos(\theta^{2})}

Can anyone point out my mistake?

I have attached my working out in a picture format below (may need to enlarge it)

thanks
 

Attachments

  • tute3q2cp1_smallres.jpg
    tute3q2cp1_smallres.jpg
    34 KB · Views: 498
  • tute3q2cp2_smallres.jpg
    tute3q2cp2_smallres.jpg
    28.6 KB · Views: 501
Physics news on Phys.org
JamesGoh said:
For the following problem

\frac{dw}{d\theta}=\theta w^{2}sin(\theta^{2}), w(0)=1

I am not able to obtain the solution

w=\frac{2}{1+cos(\theta^{2})}

Can anyone point out my mistake?

I have attached my working out in a picture format below (may need to enlarge it)

thanks

sin(θ^{2}) ≠ sin^{2}(θ)

Be careful where you put the exponents when using trig functions. It makes a difference for integrating and differentiating as well.
 
sorry wrong piece of paper.

Please look at this new attachment and inform me of my error
 

Attachments

  • tute3q2c_small.jpg
    tute3q2c_small.jpg
    38.2 KB · Views: 467
You forgot the arbitrary constant.
 
nothing wrong with the u-substitution and integration, however on the right side toward the bottom of your page, one step reads:

1/w = 2/(cos(Θ2)) + C

What I suggest:

1). ALWAYS isolate the function variable first (in this case w(Θ)) before solving for the arbitrary constant (in this case C).

In other words - rather than what it reads now: 1/w(Θ) = ...

rearrange the equation such that it reads w(Θ) = ...

THEN solve with the initial value w(0) = 1

2). AlSO (and more importantly) keep in mind that any value multiplied by an arbitrary constant is still the value of the arbitrary constant (e.g. 2 * C = C)

So putting these two suggestions together, rewrite 1/w = 2/(cos(Θ2)) + C such that it reads 1/w(Θ) = ... then one ENTIRE fraction. Then flip the fractions on either side so that it reads w(Θ) = ...

THEN solve with the initial condition w(0) = 1.

hope this helps
 
Last edited:

Similar threads

  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 36 ·
2
Replies
36
Views
5K
Replies
3
Views
1K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
Replies
5
Views
2K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 92 ·
4
Replies
92
Views
8K