Download in logarithmic form

Unit 11, Part 2:
Day 1:
Introduction To
Logarithms
Logarithms were originally
developed to simplify complex
arithmetic calculations.
They were designed to transform
multiplicative processes
into additive ones.
(We use them now to help us solve exponential equations,
when the “x” is a part of the exponent.)
If at first this seems like no big deal,
then try multiplying
2,234,459,912 and 3,456,234,459.
Without a calculator !
Using logarithms, the first number in log form(base 10)
has a log value of about 9.349. The second has a log value of
about 9.539. ADDING the two values together gets us a log
value of 18.888
Converting it back into a normal form (using a table of log
values), we get about
7,726,805,851,000,000,000.
Clearly, it was a lot easier to add
versions of these two numbers and then converting it.
Today of course we have calculators
and scientific notation to deal with such
large numbers.
So at first glance, it would seem that
logarithms have become obsolete.
Indeed, they would be obsolete except for one
very important property of logarithms.
It is called
The Power Property and we
will learn about it in another lesson.
For now we need only to observe that
it is an extremely important part
of solving exponential equations.
Our first job is to
try to make some
sense of
logarithms.
Our first question then
must be:
What is a logarithm ?
Of course logarithms have
a precise mathematical
definition just like all terms in
mathematics. So let’s
start with that.
Definition of Logarithm
Suppose b>0 and b≠1,
there is a number ‘p’
such that:
logb n  p if and only if b  n
p
Also, a logarithmic function is the
INVERSE of an exponential function.
The first, and perhaps the
most important step, in
understanding logarithms is
to realize that they always
relate back to exponential
equations.
You must be able to convert
an exponential equation into
logarithmic form and vice
versa.
So let’s get a lot of practice with this !
This is how we convert from one form to another…
LOG FORM ↔ EXPONENTIAL FORM
log base (value) = exponent ↔ base exponent = value
The Log and
Loop Trick…
Example 1:
Write 2  8 in logarithmic form.
3
Solution:
log2 8  3
We read this as: ”the log
base 2 of 8 is equal to 3”.
Example 1a:
Write 4  16 in log arithmic form.
2
Solution:
log4 16  2
Read as: “the log
base 4 of 16 is
equal to 2”.
Example 1b:
1
Write 2 
in log arithmic form.
8
3
Solution:
1
log2   3
8
1
Read as: "the log base 2 of is equal to -3".
8
NOTE: Logarithms can be equal to negative numbers BUT…
The base of a logarithm cannot be negative…
NOR can we take the log of a negative number!!
Okay, so now it’s time for
you to try some on your
own.
1. Write 7  49 in log arithmic form.
2
Solution: log 7 49  2
2. Write 5  1 in log arithmic form.
0
Solution:
log5 1  0
2
3. Write 10
Solution:
1

in log arithmic form.
100
1
log10
 2
100
1
2
4. Finally, write 16  4
in log arithmic form.
Solution:
1
log16 4 
2
It is also very important to be
able to start with a logarithmic
expression and change this
into exponential form.
This is simply the reverse of
what we just did.
Example 1:
Write log3 81  4 in exp onential form
Solution:
3  81
4
Example 2:
1
Write log2   3 in exp onential form.
8
Solution:
3
2
1

8
Okay, now you try these next three.
1. Write log10 100  2 in exp onential form.
Solution:
10  100
2
1
2. Write log5
  3 in exp onential form.
125
Solution:
1
5 
125
3
1
3. Write log27 3 
in exp onential form.
3
Solution:
1
3
27  3