1. Day 109 - Exploring Exponential Models

2. What is an exponential equation?
An exponential equation has the general form
y = abx

3. Growth Factor, Decay Factor
When 0 < b < 1, b is the decay factor
Given the general form y = abx
When b > 1, b is the growth factor

4. Growth or Decay???
Growth
Decay
Growth
Decay
Growth
Decay

5. Graphs of Exponential Functions

6. In this example, the asymptote is the x axis.
What is an asymptote?
“Walking halfway to the wall”
An Asymptote is a line that a graph approaches as x or y increases in absolute value.
In this example, the asymptote is the x axis.

7. Graphing Exponential Functions
Y=100(.5)x -3 -2 -1 1 2 3
Complete the table using the integers -3 through 3 for x.

8. Let’s graph one togetherX
.5x
Y=100(.5)x -3
.5-3
800 -2
.5-2
400 -1
.5-1
200
.50
100 1
.51 50 2
.52 25 3
.53
12.5

9. Let’s try one X
.5x
y=2(.5)x -3
.5-3 16 -2
.5-2 8 -1
.5-1 4
.50 2 1
.51
.52
0.5 3
.53
0.25
Complete the table using the integers -3 through 3 for x. Then graph the function.

10. Let’s try one X
.5x
y=2(.5)x -3
.5-3 16 -2
.5-2 8 -1
.5-1 4
.50 2 1
.51
.52
0.5 3
.53
0.25

11. Let’s try one X
10x
y=5(10)x -3 -2 -1 1 2 3
Complete the table using the integers -3 through 3 for x. Then graph the function.

12. Let’s try one X
10x
y=5(10)x -3
10-3
0.005 -2
10-2
0.05 -1
10-1
0.5
100 5 1
101 50 2
102
500 3
103
5000

13. Writing Exponential Equations
Find the exponential equation passing through the points (3,20) and (1,5).
Start with the general form.
Choose a point. Substitute for x and y using (3, 20)
Solve for a
Substitute x and y using (1, 5) and a using
Division property of exponents

14. Writing Exponential Equations
Find the exponential equation passing through the points (3,20) and (1,5).
Simplify
Solve for b
Go back to the equation for a; substitute in b and solve for a

15. Writing Exponential Equations
Find the exponential equation passing through the points (3,20) and (1,5).
Going back to the general form, substitute in a and b
The exponential equation passing through the points (3,20) and (1,5) is

16. Let’s Try One
Find the exponential equation passing through the points (2,4) and (3,16).
Start with the general form.
Choose a point. Substitute for x and y using (2, 4)
Solve for a
Substitute x and y using (3, 16) and a using
Division property of exponents

17. Writing Exponential Equations
Simplify
Solve for b
Go back to the equation for a; substitute in b and solve for a
Going back to the general form, substitute in a and b
The exponential equation passing through the points (2,4) and (3,16) is

18. Putting it all together . . .
Find the equation of the exponential function that goes through (1,6) and (0,2). Graph the function.

20. x 3x
y=2(3)x -3
3-3
0.074 -2
3-2
0.22 -1
3-1
0.66 30 2 1 31 6 32 18 3 33 54

21. Modeling Growth with an Exponential Equation
The growth factor can be found in word problems using b = 1 + r
where r = rate or amount of increase. You can substitute your new b into your general equation to find the exponential function.

22. EX- a guy puts $1000 into a simple 3% interest account
EX- a guy puts $1000 into a simple 3% interest account. What is the exponential equation?
r = rate 3% (write as 0.03)
b = 1 + r = 1.03
x = time
a = amount put into the account ($1,000)

23. a = the original number in the colony (1,000 bacteria )
EX – a colony of 1000 bacteria cells doubles every hour. What is the exponential equation?
r = 1 (why not 2?)
b = r + 1 = 2
x = time (in hours)
a = the original number in the colony (1,000 bacteria )
b = r + 1, where r is the amount of increase. We are increasing by 100% each time something doubles, so r = 1

24. EX- a $15000 car depreciates at 10% a year
EX- a $15000 car depreciates at 10% a year. What is the exponential equation?
r = - 10% (the car is worth 10% less each year)
b = 1 - r = 1 – 0.1 = 0.9
x = time (in years)
a = amount put into the account ($15,000)