• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Finding Logarithmic equations

  • Thread starter Cummings
  • Start date
52
0
Well then i am new here and can tell i am going to spend quite some time here aswell :)

I need some help with Finding Logarithmic/exponential equations from points on a graph.

In general, says my math book, you need 2 points on a graph to find 2 unknowns.

fair enough..i covered that all fine.
but now we have 3 unknowns..with 2 points and 1 asymptote

i know an asymptote is a line that the graph approaches but never meets...
i can therefore pick a point and with quite a large amount of accuracy say that that is the 3rd point
for example
say if the graph is traveling close to the asymptote, x = 3 at anything above y = 8 then i just pick a y value(above 8) and assume the x value to be 3.
and i would have a third point....i dont haveto get the answer to 100 decimal places so assuming the x value to be 3 would make quite an accurate answer.

anyways..i have uploaded the question for u to look at...its question 8
 

Attachments

dav2008

Gold Member
608
1
where is the problem?
 
333
1
Hello Cummings, and welcome to the forums
In general, says my math book, you need 2 points on a graph to find 2 unknowns.
That is not totally right, it mostly depends on what variables you are talking about, and wether or not you know the family of the graph that is drawn.

but now we have 3 unknowns..with 2 points and 1 asymptote
Again, it depends, on the unknowns.
say if the graph is traveling close to the asymptote, x = 3 at anything above y = 8 then i just pick a y value(above 8) and assume the x value to be 3. I dont haveto get the answer to 100 decimal.
If this is everyday math, then (although i do not understand what is your problem about), yes.
But in math, you either have to take everything to be accurate, or you have to calculate the accuracy, and make sure it lies within the the range required by the problem.
anyways..i have uploaded the question for u to look at...its question 8
I am sorry, i don't seem to be able to find the file anywhere !

Try to make your question clearer, and i am sure we will be able to answer you better.
 

Hurkyl

Staff Emeritus
Science Advisor
Gold Member
14,845
17
Points aren't the only source of information.

For instance, in problem #8, you know what the asymptote of the graph is. You also know (I presume) how to find the asymptote of a logarithmic graph from its equation. You can use this knowledge in order to write one of the three equations you need to compute A, b, and c. (The other two equations come from the given points)
 
52
0
https://www.physicsforums.com/attachment.php?s=&postid=18189

i uploaded it before..unless u cant see it....

its question 8...
the equation is Alog[base e](x-b)+c
and i have the points (4,9) and (8,-6) and the asymptote x = 3
I need to find A, b and c
 

Tom Mattson

Staff Emeritus
Science Advisor
Gold Member
5,475
20
Got it.

So, 1st thing: What does the asymptote tell you?
Well, you know that ln(0)=[oo], so you can find b from that.

Next, you use the two points given to find the other two constants.

Can you try that? If you get stuck, post what you've got and we'll help.
 
52
0
Hurkyl, your right...

if the asymptote is at X = 3 the graph has been moved 3 units to the right... the value of b is what moves it so b must be positive and b must equal 3.

now we have eliminated b..we can use the 2 given points and the new equation y = Alog[base e](x-3)+c to find the 2 remaining points.

case closed..thanks Hurkyl.
 

Related Threads for: Finding Logarithmic equations

  • Posted
Replies
7
Views
2K
Replies
7
Views
2K
Replies
4
Views
2K
Replies
2
Views
524
Replies
5
Views
316
  • Posted
Replies
7
Views
2K
  • Posted
Replies
1
Views
2K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top