Differential Equations : Differential Equations

Study concepts, example questions & explanations for Differential Equations

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Separable Variables

Solve the given differential equation by separation of variables.

\(\displaystyle x\frac{dy}{dx}=6y\)

Possible Answers:

\(\displaystyle y=6cx\)

\(\displaystyle y=6x\)

\(\displaystyle y=cx^6\)

\(\displaystyle y=x^6\)

\(\displaystyle y=cx-6\)

Correct answer:

\(\displaystyle y=cx^6\)

Explanation:

To solve this differential equation use separation of variables. This means move all terms containing \(\displaystyle y\) to one side of the equation and all terms containing \(\displaystyle x\) to the other side.

\(\displaystyle x\frac{dy}{dx}=6y\)

First, multiply each side by \(\displaystyle dx\).

\(\displaystyle xdy=6ydx\)

Now divide by \(\displaystyle y\) on both sides.

\(\displaystyle \frac{xdy}{y}=6dx\)

Next, divide by \(\displaystyle x\) on both sides.

\(\displaystyle \frac{dy}{y}=\frac{6dx}{x}\)

From here take the integral of both sides. Remember rules for logarithmic functions as they will be used in this problem.

\(\displaystyle \\\int\frac{dy}{y}=\int\frac{6dx}{x} \\\\\ln y=6\ln x \\\\e^{\ln y}=e^{6\ln x} \\\\y=e^{\ln x^6} \\\\y=cx^6\) 

Example Question #2 : First Order Differential Equations

Solve the following differential equation

\(\displaystyle \frac{dy}{dx}=3x^2y\)

Possible Answers:

\(\displaystyle ln(y)=e^x^{^{3}}\)

\(\displaystyle y=Ce^x^{^{3}}\)

\(\displaystyle y=e^x^{^{3}}\)

\(\displaystyle ln(y)=x^3\)

\(\displaystyle y=e^x^{^{3}+C}\)

Correct answer:

\(\displaystyle y=Ce^x^{^{3}}\)

Explanation:

So this is a separable differential equation. The first step is to move all of the x terms (including dx) to one side, and all of the y terms (including dy) to the other side. 

So the differential equation we are given is:  \(\displaystyle \frac{dy}{dx}=3x^2y\)

Which rearranged looks like: \(\displaystyle \frac{dy}{y}=3x^2dx\)

At this point, in order to solve for y, we need to take the anti-derivative of both sides: \(\displaystyle \int \frac{dy}{y}=\int 3x^2dx\)

Which equals: \(\displaystyle ln(y)=x^3+C\)

And since this an anti-derivative with no bounds, we need to include the general constant C

So, solving for y, we raise e to the power of both sides:

\(\displaystyle e^{ln(y)}=e^{x^3+C}\) which, simplified gives us our answer:

\(\displaystyle y=Ce^{x^3}\)

Example Question #1 : Separable Variables

 

 

Solve the following separable differential equation: \(\displaystyle y' = 5yt^4\) with \(\displaystyle y(0) = 3\).

Possible Answers:

\(\displaystyle y = 3e^{t^5}\)

\(\displaystyle y = 4e^{t^4}\)

\(\displaystyle y = 5e^{t^5}\)

\(\displaystyle y = 3e^{3t^5}\)

\(\displaystyle y = 3e^{t^4}\)

Correct answer:

\(\displaystyle y = 3e^{t^5}\)

Explanation:

The simplest way to solve a separable differential equation is to rewrite \(\displaystyle y'\) as \(\displaystyle \frac{\mathrm{d} y}{\mathrm{d} t}\) and, by an abuse of notation, to "multiply both sides by dt". This yields

 \(\displaystyle dy = 5yt^5dt\).

Next, we get all the y terms with dy and all the t terms with dt and integrate. Thus,

\(\displaystyle \int \frac{dy}{y} = \int 5t^4dt\) 

\(\displaystyle ln|y| +K = t^5 +C\)

Combining the constants of integration and exponentiating, we have 

\(\displaystyle |y| = e^{t^5 + C}\)

\(\displaystyle y = \pm e^Ce^{t^5}\)

The plus minus and the \(\displaystyle e^C\) can be combined into another arbitrary constant, yielding \(\displaystyle y = Ce^{t^5}\).

Plugging in our initial condition, we have

\(\displaystyle 3 = C\)

and

\(\displaystyle y = 3e^{t^5}\)

Example Question #1 : Separable Variables

Solve the general solution for the ODE:

\(\displaystyle \frac{dy}{dx}=x^8y\)

Possible Answers:

\(\displaystyle \ln(y) = x^8+C\)

 

where C is an arbitrary constant

\(\displaystyle y = \frac{x^9}{9}+C\)

 

where C is an arbitrary constant

\(\displaystyle \ln(y) = e^\frac{x^9}{9}+C\)

where C is an arbitrary constant

\(\displaystyle \ln(y) = \frac{x^9}{9}+C\)

 

where C is an arbitrary constant

Correct answer:

\(\displaystyle \ln(y) = \frac{x^9}{9}+C\)

 

where C is an arbitrary constant

Explanation:

First the differential equation can be separated to:

 \(\displaystyle \frac{dy}{y} = x^8dx\)

 

And then integrated simply to:

\(\displaystyle \ln(y) = \frac{x^9}{9}+C\)

Example Question #1 : First Order Differential Equations

Is the following differential equation separable? If So, how does the equation separate?

\(\displaystyle \frac{dy}{dt}= e^{t+y}\)

Possible Answers:

This differential equation is autonomous and is therefore not separable.

The differential equation is separable and becomes:

\(\displaystyle e^y dy = \frac{e^t}{dt}\)

The differential equation is separable and becomes: 

\(\displaystyle \frac{dy}{e^y} = e^t dt\)

The differential equation is separable and becomes: 

\(\displaystyle e^y dy =e^t dt\)

Correct answer:

The differential equation is separable and becomes: 

\(\displaystyle \frac{dy}{e^y} = e^t dt\)

Explanation:

Using exponential rules, we note that \(\displaystyle e^{t+y}\) becomes \(\displaystyle e^t e^y\). Meaning that the

differential equation is equivalent to:

 \(\displaystyle \frac{dy}{dt} = e^t e^y\) 

 

which by separation of variables is:

 \(\displaystyle \frac{dy}{e^y}= e^tdt\) 

 

Example Question #1 : Separable Variables

Is the following differential equation separable, if so, how does the equation separate?

\(\displaystyle y'= 3cos(ty)\)

Possible Answers:

The Differential equation is separable and becomes:

\(\displaystyle cos(t)dt = cos(y)dy\)

The differential equation is not separable.

The Differential equation is separable and becomes:

\(\displaystyle 3cos(t)dt = sin(y)dy\)

The Differential equation is separable and becomes:

\(\displaystyle 3cos(t)dt = 3cos(y)dy\)

Correct answer:

The differential equation is not separable.

Explanation:

The differential equation \(\displaystyle y'= 3cos(ty)\) cannot be written as \(\displaystyle y' = f(t)g(y)\) and is therefore not separable.

Example Question #3 : Separable Variables

Solve the given differential equation by separation of variables.

\(\displaystyle x\frac{dy}{dx}=6y\)

Possible Answers:

\(\displaystyle y=cx^6\)

\(\displaystyle y=cx-6\)

\(\displaystyle y=x^6\)

\(\displaystyle y=6cx\)

\(\displaystyle y=6x\)

Correct answer:

\(\displaystyle y=cx^6\)

Explanation:

To solve this differential equation use separation of variables. This means move all terms containing \(\displaystyle y\) to one side of the equation and all terms containing \(\displaystyle x\) to the other side.

\(\displaystyle x\frac{dy}{dx}=6y\)

First, multiply each side by \(\displaystyle dx\).

\(\displaystyle xdy=6ydx\)

Now divide by \(\displaystyle y\) on both sides.

\(\displaystyle \frac{xdy}{y}=6dx\)

Next, divide by \(\displaystyle x\) on both sides.

\(\displaystyle \frac{dy}{y}=\frac{6dx}{x}\)

From here take the integral of both sides. Remember rules for logarithmic functions as they will be used in this problem.

\(\displaystyle \\\int\frac{dy}{y}=\int\frac{6dx}{x} \\\\\ln y=6\ln x \\\\e^{\ln y}=e^{6\ln x} \\\\y=e^{\ln x^6} \\\\y=cx^6\) 

Example Question #2 : Separable Variables

Solve the following initial value problem: \(\displaystyle 6y' = y^4\sin(t)\)\(\displaystyle y(0) = 1\).

Possible Answers:

\(\displaystyle \left(\frac{1 + 2\cos(t)}{2}\right)^{-2/3}\)

\(\displaystyle \left(\frac{1 + \cos(t)}{2}\right)^{-1/3}\)

\(\displaystyle \left(\frac{\sin(t) - 1}{2}\right)^{1/3}\)

\(\displaystyle \left(\frac{1 - \sin(t)}{2}\right)^{-1/3}\)

\(\displaystyle \left(\frac{1 - 2\sin(t)}{2}\right)^{-2/3}\)

Correct answer:

\(\displaystyle \left(\frac{1 + \cos(t)}{2}\right)^{-1/3}\)

Explanation:

This is a separable differential equation. The simplest way to solve this is to first rewrite \(\displaystyle y'\) as \(\displaystyle \frac{\mathrm{d} y}{\mathrm{d} t}\) and then by an abuse of notation to "multiply both sides by dt." This yields \(\displaystyle 6dy = y^4\sin(t)dt\). Then group all the y terms with dy and integrate, getting us to \(\displaystyle \int \frac{6}{y^{4}}dy = \int \sin(t)dt = -\frac{2}{y^3} = -\cos(t) + C\). Solving for y, we have \(\displaystyle y = \left(\frac{\cos(t)}{2} + C\right)^{-1/3}\). Plugging in our condition, we find \(\displaystyle 1 = \left( \frac{1}{2} + C\right )^{-1/3}\). Raising both sides to the power of -1/3, we see \(\displaystyle 1 = \frac{1}{2} + C; C = \frac{1}{2}\). Thus, our final solution is \(\displaystyle y = \left(\frac{1 + \cos(t)}{2}\right)^{-1/3}\)

Example Question #3 : Separable Variables

Solve the following equation

\(\displaystyle \frac{1}{2}\frac{dy}{dx} = \sqrt{y+1}\cos(x) \ \ \ y(\pi) = 0\)

Possible Answers:

\(\displaystyle y = (\sin{x} + 1)^2 - 1\)

\(\displaystyle y = (\cos{x} + 1)^2\)

\(\displaystyle y = (\sin{x} + 1)^2\)

\(\displaystyle y = (\cos{x} + 1)^2 - 1\)

\(\displaystyle y = (\sin{x} - 1)^2 - 1\textup{}\)

Correct answer:

\(\displaystyle y = (\sin{x} + 1)^2 - 1\)

Explanation:

This is a separable ODE, so rearranging

\(\displaystyle \frac{1}{\sqrt{y+1}} \ dy = 2\cos{x} \ dx\)

Integrating

\(\displaystyle 2(y+1)^{1/2} = 2\sin{x} + C\)

Plugging in the initial condition and solving gives us

\(\displaystyle C=2\)

Solving for \(\displaystyle y\) gives us

\(\displaystyle 2(y+1)^{1/2} = 2\sin{x} + 2\)

\(\displaystyle (y+1) = (\sin{x} + 1)^2\)

\(\displaystyle y = (\sin{x} + 1)^2 - 1\)

Example Question #1 : Linear & Exact Equations

Find the general solution of the given differential equation and determine if there are any transient terms in the general solution.

\(\displaystyle x\frac{dy}{dx}+3y=x^4-x\)

Possible Answers:

\(\displaystyle \\y=\frac{1}{7}x^4-\frac{1}{4}x+cx^{-3} \\\\\text{Transient Term}=cx^{-3}\)

\(\displaystyle \\y=\frac{1}{7}x^3-\frac{1}{4}x+cx^{-3} \\\\\text{Transient Term}=cx^{-3}\)

\(\displaystyle \\y=\frac{1}{7}x^4-\frac{1}{4}x+cx^{-3} \\\\\text{Transient Term}=cx^{3}\)

\(\displaystyle \\y=\frac{1}{7}x^4-\frac{1}{4}x^2+cx^{-3} \\\\\text{Transient Term}=cx^{-3}\)

\(\displaystyle \\y=\frac{1}{7}x^4-\frac{1}{4}x^3+cx^{-3} \\\\\text{Transient Term}=cx^{-3}\)

Correct answer:

\(\displaystyle \\y=\frac{1}{7}x^4-\frac{1}{4}x+cx^{-3} \\\\\text{Transient Term}=cx^{-3}\)

Explanation:

First, divide by \(\displaystyle x\) on both sides of the equation.

\(\displaystyle \\\frac{dy}{dx}+\frac{3y}{x}=\frac{x^4}{x}-\frac{x}{x} \\\\\frac{dy}{dx}+\frac{3y}{x}=x^3-1\)

Identify the factor \(\displaystyle p(x)\) term.

\(\displaystyle p(x)=\frac{3}{x}\)

Integrate the factor.

\(\displaystyle e^{\int p(x)}=e^{\frac{3}{x}}=e^{3\ln x}=e^{\ln x^3}=x^3\)

Substitute this value back in and integrate the equation.

\(\displaystyle \\\int \frac{d}{dx}[x^3y]=\int x^3(x^3-1)dx \\\\x^3y=\int x^6-x^3dx \\\\x^3y=\frac{x^7}{7}-\frac{x^4}{4}+c\)

Now divide by \(\displaystyle x^3\) to get the general solution.

\(\displaystyle \\y=\frac{1}{7}\frac{x^7}{x^3}-\frac{1}{4}\frac{x^4}{x^3}+\frac{c}{x^3} \\\\y=\frac{1}{7}x^4-\frac{1}{4}x+cx^{-3}\)

The transient term means a term that when the values get larger the term itself gets smaller. Therefore the transient term for this function is \(\displaystyle cx^{-3}\).

Learning Tools by Varsity Tutors