Lesson 3



DO NOT WRITE ON THIS SHEET!

You are EXPECTED TO TAKE NOTES IN YOUR NOTEBOOK! A copy of this lesson will be posted on Mrs. Ghillany’s web site if you want to print a copy for yourself.

Unit e Ln 2: Exponential vs. Logarithmic expressions

Logarithms are simply another way to write exponents. Exponential and logarithmic functions are inverses of each other.

Logarithms are the "opposite" of exponentials, just as subtraction is the opposite of addition and division is the opposite of multiplication

Definition of Logarithmic Function:

|—The Relationship—         |

|  y = bx |

|  |

|..............is equivalent to............... |

|(means the exact same thing as) |

|logb(y) = x  |

| |

logb(y) = x  because y = bx

(Read as “log base b of y equals x”) b is the base in the exponential equation, and it is the base of

the logarithm as well.

Exponential function is solved for ‘y’ and logarithmic function is solved for ‘x’

Answer to exponential equation is the outcome/ final answer , but the answer to logarithmic equation is the exponent ‘x’ !

CONVERTING BETWEEN LOGS AND EXPONENTIAL EXPRESSIONS:

Convert the following functions into either exponential or logarithmic form:

Example 1 : Change each logarithmic expression to an exponential expression.

1. log3 27 = 3 2. log36 6 = 1 / 2 3. log2 (1 / 8) = -3 4. log8 2 = 1 / 3

TURN OVER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Solutions

1.     The logarithmic form log3 27 = 3 is equivalent to the exponential form 27 = 33

2.     The logarithmic form log36 6 = 1 / 2 is equivalent to the exponential form 6 = 361/2

3.     log2 (1 / 8) = -3 in exponential form is given by 1 / 8 = 2-3

4.     log8 2 = 1 / 3 in exponential form is given by 2 = 81/3

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Example 2 : Change each exponential expression to logarithmic expression.

1. 34 = 81 2. 41/2 = 2 3. 3-3 = 1 / 27 4. 103 = 1000

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Solutions

1.     The exponential form 34 = 81 is equivalent to the logarithmic form 4 = log3 81

Since 3 is the base and 4 is the exponent, we will have log base 3 of 81 equal to 4.

2.     The exponential form 41/2 = 2 is equivalent to the logarithmic form 1 / 2 = log4 2

3.     3-3 = 1 / 27 in logarithmic form is given by -3 = log3 (1/ 27)

4.     103 = 1000 in logarithmic form is given by 3 = log10 1000

Evaluating Logs with the Definition of Logarithm:

Logarithm with base 10 such as [pic] is called common logarithm and is written as [pic]

(base 10 is “invisible”) Common Logarithm can be evaluated on the calculator. NOTE: LOG key on your calculator indicates common log with base 10 only! If you press LOG on your calculator, calculator is using 10 as the base!

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Examples: Evaluate the following functions using the definition of logs.

1. [pic] 2. [pic] 3. [pic] 4. [pic]

5. log5(25). 6. log2(8). 7. log64(4).    8. log6(6). 9. log4(–16). 10. log2(0).

SOLUTIONS. NOTE: ANSWER TO LOGARITHM is the POWER/EXPONENT !

1. [pic] 2. [pic] 3. [pic] 4. [pic]

base = 2 base = 3 base = 4 base = 10

2 to what power is 32? 3 to what power is 1? 4 to what power is 2? 10 to what power is 1/100?

[pic] [pic] [pic] [pic]

5. log5(25). 6. log2(8). 7. log64(4).    8. log6(6). 9. log4(–16). 10. log2(0).

[pic] [pic] [pic] [pic] [pic][pic]

It is not possible t raise base to power and get

an answer < 0!!! Log argument must be >0!!!

DO NOT WRITE ON THIS SHEET!

You are EXPECTED TO TAKE NOTES IN YOUR NOTEBOOK! A copy of this lesson will be posted on Mrs. Ghillany’s web site if you want to print a copy for yourself.

Important Logarithmic Identities (rules that are always true and are VERY IMPORTANT to understand/remember):

With your partner, carefully go over the following identities and come up with examples using nUmbers, to understand why these rules are always true!

[pic] because [pic]

[pic] because [pic]

[pic] because [pic]

[pic] [pic] there is no way we can raise a positive base to some power

and end up with zero.

[pic] [pic] ( x = y

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Evaluating Logarithms that do not simplify easily

Last class, yesterday, you learned how to convert between exponential expressions and logarithmic expressions, and you also learned how to evaluate simple logarithms. For example:

1. [pic] 2. [pic] 3. [pic] 4. [pic]

base = 2 base = 3 base = 4 base = 10

2 to what power is 32? 3 to what power is 1? 4 to what power is 2? 10 to what power is 1/100?

[pic] [pic] [pic] [pic]

However, not all logarithms simplify easily.

For instance, try to evaluate [pic].

Base is 3. 3 to what power is equal to 8??? The answer/exponent here is not an integer value.

There is a formula that you can use with logarithms that do not simplify easily.

TURN OVER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Change of Base Formula: [pic]

The argument of the logarithm stays the same ‘x’ for both, but we change both logs into the logarithms with common base. The best base value to use is base = 10, because any calculator can evaluate common logarithm with base 10. a =10

Examples: Evaluate the following expressions, using the change of base formula.

a. [pic] b. [pic] c. [pic]

[pic] [pic] [pic]

Hints:

base value goes to the denominator “bottom” of the formula

using base 10 makes it easy to evaluate on the calculator

make sure you close the parentheses at the end of the argument of each logarithm

always give one or two decimal places for your final answer!

loglogloglog loglogloglogloglogloglogloglogloglogloglogloglogloglogloglogloglogloglogloglogloglog

Mixed Practice: Evaluate the following expressions/equations, using the definitions and properties of logs.

1. [pic] 2. [pic] 3. [pic] 4. [pic]

5. [pic] 6. [pic] 7. [pic] 8. [pic]

9. [pic] 10. [pic]=x 11. [pic] 12. [pic]

13. [pic] 14. [pic] 15. [pic] 16. [pic]

17. [pic] 18. [pic] 19. [pic] 20. [pic]

Expect THESE problems on your Quiz on Thursday!!!

OTL provides additional practice and answers!

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