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Solve log 3 log10 y = -2

  • Thread starter DeanBH
  • Start date
  • #1
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how do logs work when it's like this.


3 log10 y = -2

log10 y = a
10^a = y

so it's 10^-2 = y

but where does the 3 go, and why does it go there. i'm not sure.
 

Answers and Replies

  • #2
1,752
1
FFR (for future reference) :p ln is base e and log by itself, is implied base 10.

Power Rule: [tex]\log y^3=-2 \leftrightarrow 3\log y=-2[/tex]

Logarithmic to Exponential form: [tex]\log_a B=m \leftrightarrow a^m=B[/tex]
 
Last edited:
  • #3
82
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so

3logy = -2
logy^3 = -2
log10^-2 = y^3
y=10^(-2/3)
 
  • #4
1,752
1
Your third line is wrong, but your final answer is correct. I'm not sure what you were doing, but you wouldn't take the log of the right side then drop the log on the left.
 
Last edited:
  • #5
HallsofIvy
Science Advisor
Homework Helper
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Another way to do that problem is to just divide both sides by 3 at the start:
3 log y= -2 so log y= -2/3. Now, the example you showed says that y= 10-2/3 as before.
 
  • #6
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Your third line is wrong, but your final answer is correct. I'm not sure what you were doing, but you wouldn't take the log of the right side then drop the log on the left.
should be 10^-2 = y^3 without log?
 
  • #7
cristo
Staff Emeritus
Science Advisor
8,107
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ln is base e and log by itself, is implied base 10.
Whilst your first point is always true, your second point is not. I know people (myself included) who sometimes write log(x) to be base e. One should always check the conventions of the book that one is using.
 

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