# Applying Euler's Method to Differential Equations

Instructor: Gerald Lemay

Gerald has taught engineering, math and science and has a doctorate in electrical engineering.

Having a method for solving differential equations when the standard analytic techniques don't work is a great thing. In this lesson, we'll explore one of the most intuitive numerical approaches for finding a solution to a differential equation: Euler's method.

## Differential Equations

At sometime in your life, you might find yourself solving a differential equation. It might be on the job as a scientist or an engineer, or while working on a homework problem.

A differential equation is one with derivatives in it, or the points on a graph where a function changes. We solve a differential equation by finding all the x and y values that make the equation true; plotting these points on an x-y graph leads to a curve, or the solution.

Sometimes we can identify the function for the curve, which is called the analytic solution. On other occasions, the analytic solution is difficult or impossible to find. In these cases, we can use a numerical method to find some points on or close to the curve. In this lesson, we'll explore the numerical method called Euler's method.

## Recipe for Euler's Method

When solving a differential equation, we need a place to start, or an initial condition: the value of y we call yo when x equals xo.

Now we need a direction for the next point. Enter calculus!

Imagine moving along the tangent line as xo takes a step to x1. The tangent line is one that meets but does not cross a curve at a certain point.

The slope of this line is the change in y divided by the change in x. The change in y is y1 - yo, and the change in x is x1 - xo.

The slope at xo is the derivative of y with respect to x, written as dy/dx. Thus,

To streamline things, replace x1 - xo with h:

Using some algebra, solve for y1:

Here's what the differential equation looks like in this example:

This example has an analytic solution:

Let's check the analytic solution:

• Identify the initial condition (y at x = 0): 2.
• Substitute x = 0 in the solution (y = 2x2 + 2): y = 2(0) + 2 = 2.
• The analytic solution makes the differential equation true.
• Differentiate y = 2x2 + 2 with respect to x: 4x + 0 = 4x.
• The differential equation is y ' = 4x.

This example provides us with an analytic solution to compare with Euler's method. Typically, we use a numerical method when the analytic solution does not exist.

Here's our equation for y1:

Let's relate this equation to our differential equation:

Do you recognize the y prime on the left-hand side as the dy/dx in our y1 equation?

On the right-hand side, we also have a function of x and y, which we write as f(x, y). In our example, f(x, y) is simply 4x.

Do you agree dy/dx equals f(x, y)? Our differential equation is y ' = 4x. On the left-hand side, y ' is the same as dy/dx. On the right-hand side, 4x is f(x, y). Thus, dy/dx = f(x, y).

Our next step in developing Euler's method is to write:

See how we replaced dy/dx with f(x, y)?

At x = xo, y = yo. Thus, f(x, y) can be evaluated at this point:

On the right-hand side, the subscripts are zero. But on the left-hand side, the subscript is 1.

Usually, if the subscript on the right-hand side is n, the subscript on the left-hand side is n + 1:

This last equation is the recipe for Euler's method. The next value of y is based on the current value of y, the step size h (or change in x), and the function evaluated at the current x and y values. Let's see how Euler solves the differential equation in our example.

## Application of Euler's Method

Solve the differential equation: y ' = 4x on the interval x = 1 to x = 3, given y(1) = 4.

#### Step 2: Create a Table

To keep track of the computations, we'll use a 5-table column, where each column is devoted to the values found in the Euler 'recipe.'

n xn yn f(xn, yn) h f(xn, yn)

#### Step 3: Enter the n and xn Values

n xn yn f(xn, yn) h f(xn, yn)
0 1.0 4
1 1.5
2 2.0
3 2.5
4 3.0

The first value for x is 1.0. This is our xo. In steps of h = 0.5, the next values of x are 1.5, 2.0, 2.5, and 3.0. The n values start at 0 and end at n = 4. We'll also enter the starting point: y = 4 when x = 1.0.

#### Step 4: Calculate and Plot

Here, f(x, y) = 4x. So, f(xo, yo) is 4(xo) = 4(1) = 4.

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