2. Concepts and Objectives
Exponential and Logarithmic Equations
Identify e and ln x.
Set up and solve exponential and logarithmic
equations.
3. e
Suppose that $1 is invested at 100% interest per year,
compounded n times per year.
According to the formula, the compound amount at
the end of 1 year will be
As n increases, the value of A gets closer to some fixed
number, which is called e.
e is approximately 2.718281828.
1
1
n
A
n
4. Natural Logarithms
Logarithms with base e are called natural logarithms,
since they often occur in the life sciences and economics
in natural situations that involve growth and decay.
The base e logarithm of x is written ln x.
Therefore, e and ln x are inverse functions.
ln
ln
x x
e e x
5. Exponential Equations
Example: Solve . Round to the nearest
thousandth.
If x > 0, y > 0, a > 0, and a 1, then
x = y if and only if logax = logay
2 1 2
3 0.4
x x
6. Exponential Equations
Example: Solve . Round to the nearest
thousandth.
If x > 0, y > 0, a > 0, and a 1, then
x = y if and only if logax = logay
2 1 2
3 0.4
x x
2 1 2
log3 log0.4
x x
7. Exponential Equations
Example: Solve . Round to the nearest
thousandth.
If x > 0, y > 0, a > 0, and a 1, then
x = y if and only if logax = logay
2 1 2
3 0.4
x x
2 1 2
log3 log0.4
x x
2 1 2
log3 log0.4
x x
8.
2 log3 log3 log0.4 2log0.4
x x
Exponential Equations
Example: Solve . Round to the nearest
thousandth.
If x > 0, y > 0, a > 0, and a 1, then
x = y if and only if logax = logay
2 1 2
3 0.4
x x
2 1 2
log3 log0.4
x x
2 1 2
log3 log0.4
x x
9. Exponential Equations
Example: Solve . Round to the nearest
thousandth.
If x > 0, y > 0, a > 0, and a 1, then
x = y if and only if logax = logay
2 1 2
3 0.4
x x
2 1 2
log3 log0.4
x x
2 1 log3 2 log0.4
x x
log0.
l 4
og3
2 log3 2log0.4
x x
log0.4
2 log3 2log g
0.4 lo 3
x x
10. Exponential Equations
Example: Solve . Round to the nearest
thousandth.
If x > 0, y > 0, a > 0, and a 1, then
x = y if and only if logax = logay
2 1 2
3 0.4
x x
2 1 2
log3 log0.4
x x
2 1 log3 2 log0.4
x x
2 log3 log3 log0.4 2log0.4
x x
2log3 log0.4 2log0.4 log3
x
2 log3 log0.4 2log0.4 log3
x x
11. Exponential Equations
Example: Solve . Round to the nearest
thousandth.
If x > 0, y > 0, a > 0, and a 1, then
x = y if and only if logax = logay
2 1 2
3 0.4
x x
2 1 2
log3 log0.4
x x
2 1 log3 2 log0.4
x x
2 log3 log3 log0.4 2log0.4
x x
2log3 log0.4 2log0.4 log3
x
2 log3 log0.4 2log0.4 log3
x x
2log0.4 log3
0.236
2log3 log0.4
x
12. Properties of Logs, Revisited
You could also finish this as
2log0.4 log3
2log3 log0.4
x
2
2
log0.4 log3
log3 log0.4
log 0.16 3
9
log
0.4
log0.48
log22.5
This is an
exact answer.
13. Solving a Logarithmic Equation
Example: Solve
log 6 log 2 log
x x x
14. Solving a Logarithmic Equation
Example: Solve
log 6 log 2 log
x x x
6
log log
2
x
x
x
6
2
x
x
x
6 2
x x x
2
6 2
x x x
2
6 0
x x
3 2 0
x x
3, 2
x
2
0 6
x x
If we plug in –3, x+2
is negative. We can’t
take the log of a
negative number, so
our answer is 2.
15. Solving a Base e Equation
Example: Solve and round your answer to the
nearest thousandth.
2
200
x
e
16. Solving a Base e Equation
Example: Solve and round your answer to the
nearest thousandth.
2
200
x
e
2
ln ln200
x
e
2
ln200
x
ln200
x
2.302
x
17. Solving a Base e Equation
Example: Solve
ln
ln ln 3 ln2
x
e x
18. Solving a Base e Equation
Example: Solve
ln
ln ln 3 ln2
x
e x
ln ln 3 ln2
x
x
ln
ln ln 3 ln2
x
x
e
ln ln2
3
x
x
2
3
x
x
2 6
x x
6 x
2 3
x x