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**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

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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

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**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 –

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

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**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.

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

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**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.

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

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**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

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**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

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