help!! hard logarithm problem I dont get!

A

andrea1989

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Oct 5, 2009
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okay ive been trying to get the right answer to this question for some time now. I have to enter the problem on a computer program using keyboard commands.

f(x)=12log(5x-2)

if f(x)=8 find the solution for x

I keep getting this: ((10^8(1/12)+2)/5 but the computer says it wrong...
 
andrea1989 said:
okay ive been trying to get the right answer to this question for some time now. I have to enter the problem on a computer program using keyboard commands.

f(x)=12log(5x-2)

if f(x)=8 find the solution for x

I keep getting this: ((10^8(1/12)+2)/5 <<< How do you get that?

f(x) = 12log(5x-2)

8 = 12 * log(5x-2)

log(5x-2) = 8/12

5x - 2 = 10[sup:1ci10pul]2/3[/sup:1ci10pul]

and continue.....
but the computer says it wrong...
Click to expand...
 
Let me try : Not sure : but I'll try ....

f(x) = 12log(5x-2) = 8

log(5x-2) = 2/3

log(5x - 2) = log (2/3)

5x - 2 = 2/3 .. Solve for x ??

I'm a beginner in Logarithms :wink:

Is my work correct :: Guys ::
 
Aladdin said:
Let me try : Not sure : but I'll try ....

f(x) = 12log(5x-2) = 8

log(5x-2) = 2/3

log(5x - 2) = log (2/3) << Incorrect - look at my solution
5x - 2 = 2/3 .. Solve for x ??

I'm a beginner in Logarithms :wink:

Is my work correct :: Guys ::
Click to expand...
 
f(x)=12log(5x-2)

if f(x)=8 find the solution for x

I keep getting this: ((10^8(1/12)+2)/5 but the computer says it wrong...
Click to expand...

You are likely running into an “order of operations” error. The way you’ve written it, 10^8 is being multiplied by (1/12). You also have an error in your grouping symbols: three left hand parentheses and only two right side. You need to simplify your expression and be more careful with grouping.

8/12 = 2/3
(10^(2/3) + 2)/5 = x

Hope that helps.
 
andrea1989 said:
((10^8(1/12)+2)/5 but the computer says [it is] wrong
Click to expand...


Regardless of the actual numerical value of whatever this expression is supposed to mean, there is another reason why computers will refuse to accept it.

You've typed three open parentheses, but only two close parentheses.

Parentheses ALWAYS come in pairs.

 
Subhotosh Khan said:
Aladdin said:
Let me try : Not sure : but I'll try ....

f(x) = 12log(5x-2) = 8

log(5x-2) = 2/3

log(5x - 2) = log (2/3) << Incorrect - look at my solution
5x - 2 = 2/3 .. Solve for x ??

I'm a beginner in Logarithms :wink:

Is my work correct :: Guys ::
Click to expand...
Click to expand...

So the log will be canceled by the 10 or by the base 10 ... But I think it's not requisted these days ?? Books are concentrating on Natural logarithms? isn't ?
 
Aladdin said:
log(5x-2) = 2/3 Going from this line

log(5x - 2) = log (2/3) to this line
Click to expand...


You've taken the logarithm of the righthand side only.

That's a big no-no, in algebra.

If you want to take the logarithm of one side, you must do the same on the other side. 8-)

Of course, you don't want to take the logarithm of both sides, in this case, because you'll end up with log[log(5x-2)].

 
Aladdin said:
base 10 ... But I think it's not [required] these days ?? Books are concentrating on Natural logarithms?
Click to expand...


The natural base is probably more common, in the real world, but we should be fluent in base 10, too.

Base 10 is used for things like measuring earthquake magnitudes and pH levels.

 
Re:

mmm4444bot said:
Aladdin said:
log(5x-2) = 2/3 Going from this line

log(5x - 2) = log (2/3) to this line
Click to expand...


You've taken the logarithm of the righthand side only.

That's a big no-no, in algebra.

If you want to take the logarithm of one side, you must do the same on the other side. 8-)

Of course, you don't want to take the logarithm of both sides, in this case, because you'll end up with log[log(5x-2)].
Click to expand...

Yes Mark - I've got your point - Thanks All ... Questions will come when my school starts this thursday :)
 
f(x) = 12 log(5x-2)

8 = 12 log(5x-2)

2/3 = log (5x-2)

*let (5x-2) = y

2/3 = log y

*log is log10 when it has no base number.

2/3 = log10 y

y = 10 2/3

5x-2 = 10 2/3

5x = 2 + 10 2/3

x = (2 + 10 2/3) / 5

x = 2/5 + 10 2/3 / 5

Therefore, x = 0.4 + (10 2/3 / 5)
 
Denis said:
Jonathan, that's a 2009 thread!
Click to expand...

When a thread sits 10 days with no activity, I think it's okay to post camera-ready copy.

As most posters here quickly abandon their discussion(s), I think that it's a nice thing for future readers when others take time to tidy up. :D
 
2012_jonathan_s said:
*let (5x-2) = y

2/3 = log y
Click to expand...

Hi Jonathan

If the function in this exercise were to be graphed, then y = 8 when x = 1.3283 (rounded)

Symbol y is chosen to represent the number f(x) so frequently in math that I would choose a different symbol to use above. (Just a suggestion)



*log is log10 when it has no base number.
Click to expand...

Unfortunately, this is no longer true in the real world.

Many mathematicians now use log(x) to mean the natural log of x.

Word to the wise: Understand your authors.



x = 2/5 + 10 2/3 / 5

Therefore, x = 0.4 + (10 2/3 / 5)
Click to expand...

This final answer is correct, but why did you convert 2/5 to decimal form?

If one converts to decimal form, they may as well write: x = 1.3283 (rounded)

Cheers :cool:
 
Last edited:
2012_jonathan_s said:
f(x) = 12 log(5x-2)

8 = 12 log(5x-2)

2/3 = log (5x-2)

*let (5x-2) = y

2/3 = log y

*log is log10 when it has no base number.

2/3 = log10 y

y = 10 2/3

5x-2 = 10 2/3

5x = 2 + 10 2/3

x = (2 + 10 2/3) / 5

x = 2/5 + 10 2/3 / 5

Therefore, x = 0.4 + (10 2/3 / 5)
Click to expand...


2012_jonathan_s,

you are not finished at that point. You have to check
your candidate answer in the original equation.

If it were not to check, then you might write
"no real answer" or something to that effect.
 
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