Solving 4log45 Logarithmic Problem

  • Thread starter d.smith292
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    Logarithmic
I'm going to save both of these responses for future reference.In summary, the conversation centered around solving the logarithmic equation 4log45. The participants discussed the basic rules and properties of logarithms, including the relationship between logs and exponents. They also discussed how to use a calculator to solve logarithmic equations and how to convert between different bases. The conversation ended with a recommendation to watch Khan Academy videos for a better understanding of logarithms.
  • #1
d.smith292
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Homework Statement



4log45

Homework Equations



ay = x ~~~> loga(x) = y

The Attempt at a Solution



I've separated the problem and decided to solve the logarithmic section first giving me:

4x = 5

I could move forward if I wasn't stumped on how to figure out what x is in this equation. This is probably very simple and maybe my mind isn't comprehending well this morning, but I've stared at this problem for quite some time now. I appreciate your assistance in advance.
 
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  • #2
given a number x, what is does its log represent?

knowing that you should be able to solve this immediately in your head.
 
  • #3
I'm sorry, but I'm not understanding what your trying to say. Would you mind clarifying that for me.
 
  • #4
have you tried it on a calculator yet to see what the answer is?
 
  • #5
I would type it in and work backwards, but my ti84 automatically sets the log with a base of 10 and I don't know how to change it to a base of 4. Do you know how to do this?

When I get stumped, I usually type it into the calculator and work backwards to figure it out, but in this case I can't.

How would I type this into a TI84?
 
  • #7
This is what I typed in:

4(log(5)) and this is what I got:

2.795880017

Now what?
 
  • #9
d.smith292 said:

Homework Statement



4log45

Homework Equations



ay = x ~~~> loga(x) = y

The Attempt at a Solution



I've separated the problem and decided to solve the logarithmic section first giving me:

4x = 5

I could move forward if I wasn't stumped on how to figure out what x is in this equation. This is probably very simple and maybe my mind isn't comprehending well this morning, but I've stared at this problem for quite some time now. I appreciate your assistance in advance.

Homework Statement


Homework Equations


The Attempt at a Solution


Going back to your original work:

4x = 5

take the log base 4 of both sides:

x * log44 = x = log45
 
  • #10
:biggrin:

Gotcha! I see what your saying now. If my subscript is 4 and my calculator has a default of 10, all I need to do is sub out 10 when I see 4 and I will get the same answer.

Is this correct?

For example:

4log45 would give me the same answer if I used 10log105

I've typed it in and got the right answer.

Thank you so much for your help, I really appreciate it. I promise I'm usually good at math, this one just stumped me.
 
  • #11
what you have is a kind of identity. This doesn't work in the general case as you were thinking. The teacher selected this example because you can't just enter it into the calculator to get the answer.

The identity is h/she wanted you to notice is:

base^logbase(x) = x

so it could be 10^log10(5) = 5 or e^ln(5) = 5 ...

a more complicated example might be:

8^log4(5) = ? with this variation you'd need to understand how to convert between bases
 
  • #12
Ok, so in my case with my bases being equal, my answer will always be x.

We haven't gone over this yet, but how would I convert between bases in the more complicated problem of 8^log4(5)?

Would you mind working this problem out, showing the steps so I can see?
 
  • #13
I chose poorly let's try 16^log4(5) = x

taking the log of both sides yields:

log4(5) = log16(x)

next applying the log conversion:

loga(x) = logb(x) / logb(a)

log16(x) = log4(x) / log4(16)

and since 16 is 4^2 then log4(16) = 2

log4(5) = log16(x) = log4(x) / log4(16) = log4(x) / 2

so we now have:

2 * log4(5) = log4(x)

and bringing the 2 inside

log4(5^2) = log4(x)

and lastly 5^2 = 25 and hence x=25

my original 8^log4(5) would have yielded x = 5 * sqrt(5) or 5^(3/2) since the
conversion factor log4(8) = log4(4*2)=log4(4)+log4(2) = 1 + 1/2 = 3/2
 
  • #14
Thank you, now I have a head start for next weeks lesson. I really appreciate your help with this.
 
  • #15
Basically, the first thing you should have learned about "[itex]log_a(x)[/itex]" is that it is the inverse function to [itex]f(x)= a^x[/itex]. That is, if [itex]y= a^x[/itex] then [itex]x= ln_a(y)[/itex]. Putting those together, [itex]ln_a(y)= ln_a(a^x)= x[/itex] and [itex]a^{x}= a^{ln_a(y)}= y[/itex].
 
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  • #16
Thanks for all the help you guys have provided me with. I really appreciate it. This class I'm in right now is rough because the teacher isn't explaining anything clearly. She just starts talking as if everyone knows what she's talking about and gives no explanation as to why were doing things. I'm one of those people who needs at least a little bit of a "why" to what I'm doing. I want to know where it's leading.

Anyways sorry for my rant. Thanks again.
 
  • #18
[itex]x=4^{log_45}[/itex]
Taking [itex]log_4 [/itex] both side
Whatever you do on the left of the equation, you should do the same on the right.
It doesn't matter if you add, multiply, square or any other mathematical operations.

[itex]log_4x=log_44^{log_45}[/itex]
[itex]log_4x=log_45log_44[/itex] You should remember this rule. lgx^y=ylgx
[itex]log_4x=log_45[/itex]

you can find the value of x
 
  • #19
Thanks for the Khan Academy web address, it's really simple to understand when it's explained clearly and things aren't left out. And thank you azizlwl for that great break down.
 

Related to Solving 4log45 Logarithmic Problem

What is a logarithm?

A logarithm is a mathematical function that is the inverse of an exponent. It is used to solve for an unknown exponent in an exponential equation.

What is the base of a logarithm?

The base of a logarithm is the number that is raised to a power in an exponential equation. For example, in the equation 4^x = 16, the base is 4.

How do I solve a logarithmic problem?

To solve a logarithmic problem, you can use the properties of logarithms, such as the power rule and the product rule, to simplify the equation and isolate the variable. You can also use a calculator or logarithmic tables to solve more complex problems.

What is the difference between natural logarithm and common logarithm?

The natural logarithm, denoted as ln, uses the base e (approximately equal to 2.718) while the common logarithm, denoted as log, uses the base 10. They both represent the inverse of the exponential function, but the natural logarithm is often used in mathematical and scientific calculations while the common logarithm is used more in everyday calculations.

What is the domain of a logarithmic function?

The domain of a logarithmic function is all positive real numbers. This is because a logarithm is only defined for positive numbers. The range of a logarithmic function, however, is all real numbers.

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