1. Differential Equations
Greg Kelly, Hanford High School, Richland, Washington
Adapted by: Jon Bannon, Siena College

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. 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.

5. 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

6. 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.

7. 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.)

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

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

10. 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!

11. 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”.