1. Differential Equations (10/14/13)
A differential equation is an equation which contains derivatives within it.
More specifically, it is an equation which may contain an independent variable x (or t) and/or a dependent variable y (or some other variable name), but definitely contains a derivative y ' = dy/dx (or dy/dt).
It may also contain second derivatives y '' , etc.

2. Examples of DE’s
Every anti-derivative (i.e., indefinite integral) you have solved (or tried to solve) this semester is a differential equation!
What is y if y ' = x2 – 3x + 5 ?
What is y if y ' = x / (x2 + 4)
What is y if dy/dt = e0.67t
Note that you also get a “constant of integration” in the solution.

3. New types of examples
The following is a DE of a different type since it contains the dependent variable: y ' = .08y
Say in words what this says! Sound familiar?
Note that we don’t see the independent variable at all – let’s call it t .
What is a solution to this equation? And how can we find it?

4. Clicker Question 1
What is the most general solution of the differential equation 2 dy/dt = 5 / t 4 ?
A. -5 / (3 t 3) + C
B. 5 / (3 t 3) + C
C. -5 / (6 t 3) + C
D. -1 / t 5 + C
E. -2 / t 5 + C

5. The solutions to a DE
A solution of a given differential equation is a function y which makes the equation work.
Show that y = Ae0.08t is a solution to the DE on the previous slide, where A is a constant.
Interpret this result!
Note that we are using the old tried and true method for solving equations here called “guess and check”.

6. Examples of guess and check for DE’s
Show that y = 100 – A e –t satisfies the DE y ' = y
Show that y = sin(2t) satisfies the DE d2y / dt 2 = -4y
Show that y = x ln(x) – x satisfies the DE y ' = ln(x)
Of course one hopes for better methods to solve equations, but DE’s can be very hard.

7. Clicker Question 2
Which function below satisfies the DE d2y/dx2 = 9y ?
A. y = 3ex
B. y = 3sin(x)
C. y = sin(3x)
D. y = x2 + 9x
E. y = e3x

8. Assignment for Wednesday
Read Section 9.1.
On page 584, do # 1 – 7 odd.