1. Take a logarithm of each side
EXAMPLE 2
Take a logarithm of each side
Solve 4 = 11. x
SOLUTION
4 = 11 x
Write original equation.
log = log 11 x 4
Take log of each side. 4
log 4
x =
log b = x b x
x =
log 11
log 4
Change-of-base formula x
Use a calculator.
The solution is about Check this in the original equation.
ANSWER

2. EXAMPLE 3
Use an exponential model
You are driving on a hot day when your car overheats and stops running. It overheats at 280°F and can be driven again at 230°F. If r = and it is 80°F outside,how long (in minutes) do you have to wait until you can continue driving?
Cars

3. Use an exponential model
EXAMPLE 3
Use an exponential model
SOLUTION
T = ( T – T )e T R
– rt
Newton’s law of cooling
230 = (280 – 80)e
–0.0048t
Substitute for T, T , T , and r. R
150 = 200e
–0.0048t
Subtract 80 from each side.
0.75 = e
–0.0048t
Divide each side by 200.
ln 0.75 = ln e
–0.0048t
Take natural log of each side.
– – t
In e = log e = x e x t
Divide each side by –

4. EXAMPLE 3
Use an exponential model
You have to wait about 60 minutes until you can continue driving.
ANSWER

5. GUIDED PRACTICE for Examples 2 and 3 Solve the equation. 4. 2 = 5
= 5 x
SOLUTION
2 = 5 x
Write original equation.
Log = log 5 x 2
Take log of each side. 2
log 2
x =
log b = x b x
x =
log 5
log 2
Change-of-base formula x
Use a calculator.

6. GUIDED PRACTICE
for Examples 2 and 3
The solution is about Check this in the original equation.
ANSWER

7. GUIDED PRACTICE for Examples 2 and 3 Solve the equation. 5. 7 = 15
= 15 9x
SOLUTION
7 = 15 9x
Write original equation.
Log 7 = log 15 9x 7
Take log of each side. 7
9x =
log 7
log b = x b x
9x =
log 15
log 7
Change-of-base formula x
Use a calculator.

8. GUIDED PRACTICE
for Examples 2 and 3
The solution is about Check this in the original equation.
ANSWER

9. GUIDED PRACTICE for Examples 2 and 3 Solve the equation. 6. 4e –7 = 13
SOLUTION
4e –7 = 13
–0.3x
4e =
–0.3x
4e = 20
–0.3x
e = = 5
–0.3x 20 4
log e = log 5
–0.3x e

10. The solution is about 5.365. Check this in the original equation.
GUIDED PRACTICE
for Examples 2 and 3
– 0.3x = In 5
In e = log e = x e x
Divide each side by – 0.3x.
– 0.3x =
x = = – 5.365 –
0.3
The solution is about Check this in the original equation.
ANSWER