[Jbroad572]

Ok, I decided to take you up your offer and ask for help on a specific problem. Please try to explain to me how you arrivea t your answer as well.
Here's the problem:
A model for the number of people (N) in a college community who have heard a certain rumor is N-P(1-e^-0.15d)
where P is the total population of the community and d is the number of days that have elapsed since the rumor began. In a community of 1000 students, how many days will elapse before 450 students have heard the rumor?


SO I tried it on my own and started off with
450=1000(1-e^-.15d)
I figured I had to first find solve for d and calculate the distance, which I don't know if I did right or not, but came out with 2.90 (rounded to 2 dec. places) I plugged that in and tried to start solving, but got stuck and am currently lost. I have 3 more weeks of Algebra, I would like to finish strong, so please help me to the best of your abilities.

[11011001]

Is that supposed to be:

N = P (1 - (e ^ (-.15d)) )?

in which case you would go

-((N/P) - 1) = e ^ (-.15d)

- ((450/1000) - 1) = e ^ (-.15d)

from here it is relatively simply to solve, using your logarithmic rules.

hope the hint helps.

[mindwaves]

set them equal.

divide both sides by 1000

.45 = 1- e^-0.15d

subtract 1 and take away minuses

0.55 = e^-0.15d

take natural log of both sides

-0.5978 = -0.15d

divide by -0.15 on both sides

d ~ 3.98

[maxintosh]

You need to solve the general equation for d.

N = P(1-e^-.15d)

N/P = 1-e-^.15d

-(N/P - 1) = e^-.15d

ln(-(N/P-1)) = ln(e^-.15d)

[log rule: ln of e raised to (*) is equal to (*)]
ln(-(N/P-1)) = -.15d

ln(-(N/P-1))/-.15 = d

Now you can solve for any values of N and P.

It's always a good idea to solve a general equation before you start plugging things in, or at least keep them expressed as fractions. It reduces your chances of having significant rounding errors.

[Jbroad572]

Well thanks, I would have never figured that out. It always makes sense after I see it worked out. The class I'm in, the teacher doesn't really teach. She'll do 1 or 2 problems then class is over, and the rest is up to us. I wish I would have known you guys were able to asnwer like that, I would have posted more complicated ones when I was at my wits end.
Thanks again for the explanations.
On to Logarithmic Properties.......

[maxintosh]

Originally posted by Jbroad572:
Well thanks, I would have never figured that out. It always makes sense after I see it worked out. The class I'm in, the teacher doesn't really teach. She'll do 1 or 2 problems then class is over, and the rest is up to us. I wish I would have known you guys were able to asnwer like that, I would have posted more complicated ones when I was at my wits end.
Thanks again for the explanations.
On to Logarithmic Properties.......

You're welcome. The trick with math is to keep on top of it as you learn it. It only gets a hell of a lot harder from here, if you take the Calculus track. It's a good brain workout

[Jbroad572]

I'm stuck again. This is very frustrating. The book always gives the examples for the easiest problems and then gives you the extremly hard ones. I have nothing else I can do until tomorrow, when I go to the math center. I am a very visual person and once I see it worked out, can usually take it from there. I have a test coming up on Monday Im definately not ready for. I wish I could go back to high school really quick, with the smaller classes and the one on one questions. Oh well, I'll just have to suck it up and try harder and harder.