Hello, I have a quick question.

W

WTF?

Junior Member
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Sep 16, 2005
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I'm doing a 'self-test' for math, and I stumbled upon this problem.

"Assume that bacteria decay according the exponential model y=A(.92)^x , where A cells of bacteria would become y cells x hours later."

So, given this.

1. If at some point there are 1000 bacteria, how many were there two hours earlier?

The book says it's 1181. How did they get that? Thanks for any help.
 
Hello, WTF!

I had to baby-talk my way through this one . . .

Assume that bacteria decay according the exponential model: \(\displaystyle \,y\,=\,A(0.92)^x\),
where \(\displaystyle A\) cells of bacteria would become \(\displaystyle y\) cells \(\displaystyle x\) hours later.

1. If at some point there are 1000 bacteria, how many were there two hours earlier?
Click to expand...
Let's say that at time \(\displaystyle t\), there are exactly 1000 bacteria.

Then we have: \(\displaystyle \,A(0.92)^t\:=\:1000\:\) [1]

Two hours earlier, there were: \(\displaystyle \,A(0.92)^{t-2}\) bacteria . . . How many is that?

Divide [1] by \(\displaystyle (0.92)^2\,.\,.\,.\;\frac{A(0.92)^t}{(0.92)^2}\:=\:\frac{1000}{(0.92)^2}\)

And we have: \(\displaystyle \:A(0.92)^{t-2}\:=\:\frac{1000}{(0.92)^2} \:=\:1181.47448\;\) . . . There!


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Of course, Gene (with his laser-like insight) did it in one step!
 
Hello, I have ln7

and the problem asks me to rewrite in common logarithmic form, therfore it is Loge7 ??

*base e
 
You should post new questions as a new post.
ln(7) is base e.
log(7) is base 10
log(7) = ln(7)/ln(10)
or
ln(7)= log(7)/log(e)
 
Hmmmmm, could be but the standard way for those two bases is what I showed. You could put a 2 in a square root symbol. It wouldn't be wrong, just unnecessary.
--------------------
Gene
 
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