1. 7.1 Slope Fields and Euler’s Method
AP Calculus AB
7.1 Slope Fields and Euler’s Method
Greg Kelly, Hanford High School, Richland, Washington

2. First, a little review:
Consider: or
then:
It doesn’t matter whether the constant was 3 or -5, since when we take the derivative the constant disappears.
However, when we try to reverse the operation:
Given:
find
We don’t know what the constant is, so we put “C” in the answer to remind us that there might have been a constant.

3. If we have some more information we can find C.
Given: and when , find the equation for .
This is called an initial value problem. We need the initial values to find the constant.
An equation containing a derivative is called a differential equation. It becomes an initial value problem when you are given the initial condition and asked to find the original equation.

4. Example 1. Find the general solution to the exact differential equation.
First, find the antiderivative of
This is the general solution to the Differential Equation (D.E.). If the general solution is continuous, a unique solution can be found if an initial condition if given. This gives a particular solution to the D.E.

5. Example 2. Find the particular solution to the equation.
1. Find the general solution by finding the antiderivative.
2. Substitute the initial condition.
3. Solve for C.
4. Substitute C into the general solution to get the particular solution.

6. Example 3. Your turn. Find the particular solution to the equation.

7. If a function is discontinuous, then the initial condition only pins down the continuous piece of the curve that passes through the given point. In this case the domain must be specified.
Example 4. Solve the initial value problem explicitly.
The point (1, 3) only pins down the continuous piece over a certain interval. This interval is (0, 8). When x = 1,
This interval needs to be taken into account as part of the solution. Since the point (1, 3) only takes into account positive values for x, the domain is

8. Example 4. Solve the initial value problem explicitly.

9. Sometimes we can’t easily find the antiderivative, so we need to use the Fundamental Theorem of Calculus.
Example 5. Solve the initial value problem using the Fundamental Theorem.

10. General solutions always have that + C added on as part of the function and when graphing y = f(x) + C.
The + C shifts the graph vertically by whatever the value of C is and gives us a family of functions.
Initial value problems and differential equations can be illustrated with a slope field.
Slope fields are mostly used as a learning tool and are mostly done on a computer or graphing calculator, but a recent AP test asked students to draw a simple one by hand.

11. 1 2 3 1 2 1 1 2 2 4 -1 -2 -2 -4 Draw a segment with slope of 2.
Draw a segment with slope of 2. 1 2 3 1 2
Draw a segment with slope of 0. 1 1 2
Draw a segment with slope of 4. 2 4 -1 -2 -2 -4

12. If you know an initial condition, such as (1,-2), you can sketch the curve.
By following the slope field, you get a rough picture of what the curve looks like.
In this case, it is a parabola.

13. Example 5. Construct a slope field for the differential equation and sketch a graph of the particular solution that passes through the point (2, 0).
Since (2, 0) is above what appears to be an asymptote, we only need to sketch the slope field above the asymptote to find the particular solution. 1 -1 -1 1 1 1 2 2 3 3 1 1 -1 -1 1 -2 -1 2 -3 -1 p

14. For more challenging differential equations, we will use the calculator to draw the slope field.
On the TI-89:
Push MODE and change the Graph type to DIFF EQUATIONS.
MODE
Go to: Y=
Press
and make sure FIELDS is set to SLPFLD. I
Go to: and enter the equation as: Y=
(Notice that we have to replace x with t , and y with y1.)
(Leave yi1 blank.)

15. Set the viewing
window:
WINDOW
Then draw the graph:
GRAPH

16. Be sure to change the Graph type back to FUNCTION when you are done graphing slope fields.

17. Integrals such as are called definite integrals because we can find a definite value for the answer.
The constant always cancels when finding a definite integral, so we leave it out!

18. Integrals such as are called indefinite integrals because we can not find a definite value for the answer.
When finding indefinite integrals, we always include the “plus C”.

19. Many of the integral formulas are listed on page 332
Many of the integral formulas are listed on page The first ones that we will be using are just the derivative formulas in reverse.
A graphing calculator can be used to graph the integral of a function using the numerical integration function (fnInt). or
This is extremely slow and usually not worth the trouble.
A better way is to use the calculator to find the indefinite integral and plot the resulting expression.

20. To find the indefinite integral on the TI-89, use:
The calculator will return:
Notice that it leaves out the “+C”.
Use and to put this expression
in the screen, and then plot the graph.
COPY
PASTE Y=

21. [-10,10] by [-10,10] p