Differential Equations : Undetermined Coefficients

Study concepts, example questions & explanations for Differential Equations

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Example Questions

Example Question #1 : Undetermined Coefficients

Solve the given differential equation by undetermined coefficients.

Possible Answers:

Correct answer:

Explanation:

First solve the homogeneous portion:

Therefore,  is a repeated root thus one of the complimentary solutions  is,

Now find the remaining complimentary solution .

Now solve for  and .

Where 

and 

Therefore,

Now, combine both of the complimentary solutions together to arrive at the general solution.

Example Question #2 : Undetermined Coefficients

Find the general solution for .

Possible Answers:

Correct answer:

Explanation:

This is a higher order inhomogeneous linear differential equation. Because the inhomogeneity is a cosine, we will use variation of parameters to solve it.

First, we find the characteristic equation to solve for the homogenous solution. This gives us .

This tells us that the homogeneous solution is . As neither of these overlap with our inhomogeneity, we are safe to continue without adding a factor of t.

Thus, let us guess that . Then,

 and

Plugging into the original equation, we have

Which implies that  and . Solving via substitution,

 

Thus, the particular solution is  and the overall solution is the particular plus the homogeneous.

So 

 

Example Question #3 : Undetermined Coefficients

Find the form of a particular solution to the following differential equation that could be used in the method of undetermined coefficients:

 

Possible Answers:

The form of a particular solution is    where A,B, and C are real numbers.

The form of a particular solution is    where A and B are real numbers.

The form of a particular solution is    where A,B, C, and D are real numbers.

The form of a particular solution is    where A and B are real numbers.

Correct answer:

The form of a particular solution is    where A,B, and C are real numbers.

Explanation:

We first note that the differential equation has characteristic equation 

 ,

since the roots of this characteristic polynomial are linearly independent of the forcing function   

,

we simply use undetermined coefficient combination rules to figure that the particular solution will be of form 

Example Question #4 : Undetermined Coefficients

Consider the differential equation

 

The particular solution used in undetermined coefficients will be of what form? 

Possible Answers:

The particular solution will be of form:

where A is a real number

The particular solution will be of form:

where A,B, and C are real numbers

The particular solution will be of form:

where A,B,C,D,E, and F are real numbers

The particular solution will be of form:

where A and B are real numbers

Correct answer:

The particular solution will be of form:

where A,B,C,D,E, and F are real numbers

Explanation:

We first figure that the forcing function   is linearly independent  to the homogeneous solution solved with the characteristic equation.

Therefore, using proper undetermined coefficients function rules, the particular solution will be of the form:

 

It is important to note that when either a sine or a cosine is used, both sine and cosine must show up in the particular solution guess.

Example Question #5 : Undetermined Coefficients

Solve for a particular solution of the differential equation using the method of undetermined coefficients.

  

Possible Answers:

Correct answer:

Explanation:

We start with the assumption that the particular solution must be of the form 

 .

Then we solve the first and second derivatives with this assumption, that is, 

 and .

Then we plug in these quantities into the given equation to get:

 , which solves for .

Thus, but the method of undetermined coefficients, a particular solution to this differential equation is:

 

Example Question #6 : Undetermined Coefficients

Solve the given differential equation by undetermined coefficients.

Possible Answers:

Correct answer:

Explanation:

First solve the homogeneous portion:

Therefore,  is a repeated root thus one of the complimentary solutions  is,

Now find the remaining complimentary solution .

Now solve for  and .

Where 

and 

Therefore,

Now, combine both of the complimentary solutions together to arrive at the general solution.

Example Question #7 : Undetermined Coefficients

Find the form of a particular solution to the following Differential Equation (Do NOT Solve)

 

Possible Answers:

None of the other answers.

Correct answer:

Explanation:

The form of a guess for a particular solution is 

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