log Problem

**scrible** · November 5th 2009, 04:56 PM

There is this problem that I just can not begin to solve $\log_{y}x=2$ , $5y = x + 12\log_{x}y$ this is a simultaneous equation. Can anyone help me solve this equation.

**Jameson** · November 5th 2009, 05:09 PM

Originally Posted by scrible

There is this problem that I just can not begin to solve $\log_{y}x=2$ , $5y = x + 12\log_{x}y$ this is a simultaneous equation. Can anyone help me solve this equation.

Notice that $\log_{y}(x)=2 \Rightarrow y^2=x$

Substitute $y=x^{\frac{1}{2}}$ into the second equation.

**scrible** · November 6th 2009, 05:03 PM

Well to continue this problem I am having trouble working out $5(x^\frac{1}{2})=x + 12\log_{x}(x^\frac{1}{2})$ can some one please help me with this.

**Defunkt** · November 6th 2009, 06:22 PM

Originally Posted by scrible

Well to continue this problem I am having trouble working out $5(x^\frac{1}{2})=x + 12\log_{x}(x^\frac{1}{2})$ can some one please help me with this.

Can you see why $log_x(x^{\frac{1}{2}}) = \frac{1}{2}$ ?

After you do that, you should be able to solve the rest.

**scrible** · November 7th 2009, 01:44 AM

Originally Posted by Defunkt

Can you see why $log_x(x^{\frac{1}{2}}) = \frac{1}{2}$ ?

After you do that, you should be able to solve the rest.

In the problem do I have to change the base?

**Bacterius** · November 7th 2009, 01:49 AM

Originally Posted by scrible

In the problem do I have to change the base?

It looks like you are not very familiar with logarithms. Try to read and learn the logarithm rules (yes one must know them by heart) and solve some basic but efficient problems, and then come back and try to solve this question. It will be by far easier than it is for you now.

**scrible** · November 7th 2009, 02:07 AM

Originally Posted by Bacterius

It looks like you are not very familiar with logarithms. Try to read and learn the logarithm rules (yes one must know them by heart) and solve some basic but efficient problems, and then come back and try to solve this question. It will be by far easier than it is for you now.

Do you know of any sights I can go to with examples like these? The book is not explaining it well enough for me. I learn by examples.

**Bacterius** · November 7th 2009, 02:11 AM

Originally Posted by scrible

Do you know of any sights I can go to with examples like these? The book is not explaining it well enough for me. I learn by examples.

So do I. This is why I usually refer to Wikipedia for formal lessons, and when I'm fed up with theory I can move on to the heavily-detailed examples (though sometimes I can't find any so I look for another website). And then, I can get on with the exercises in the academic department of Wikipedia when they are sufficiently reliable (let's not forget it is Wikipedia ...) and when they do exist.

For example :
- Lessons : http://en.wikipedia.org/wiki/List_of...mic_identities
- Examples : http://people.hofstra.edu/Stefan_Wan...pic1/logs.html
- Exercises : [pick any suitable website for exercises]

**scrible** · November 7th 2009, 05:28 AM

Originally Posted by Bacterius

So do I. This is why I usually refer to Wikipedia for formal lessons, and when I'm fed up with theory I can move on to the heavily-detailed examples (though sometimes I can't find any so I look for another website). And then, I can get on with the exercises in the academic department of Wikipedia when they are sufficiently reliable (let's not forget it is Wikipedia ...) and when they do exist.

For example :
- Lessons : List of logarithmic identities - Wikipedia, the free encyclopedia
- Examples : Properties of Logarithms
- Exercises : [pick any suitable website for exercises]

so far I have tried

$5(x^\frac{1}{2}) = x + 12\log_{x}(x^\frac{1}{2})$

${\frac{1}{2}}\log5x= \log x +6\log_{x}x$

and then I am stock right here. Did I do it OK so far? I want to add but I don't know if they both have to be the same base to do that. Could someone please help me with the working of this so that I can use it to solve the other problem like it in the book?

**Defunkt** · November 7th 2009, 05:34 AM

It's far simpler than you're making it out to be:

By the definition of a logarithm, $log_x(x)=1$ .

Why? -- we know that $x^{log_x(a)} = a$ . So $x^{log_x(x)} = x$ and therefore $log_x(x)=1$ .

Using this and the fact that $log_b(a^n) = n\cdot log_b(a)$ for any $a,b,n$ , we get:

$5\sqrt{x} =x + 12log_x(x^{\frac{1}{2}}) \Rightarrow 5\sqrt{x} = x + 12\cdot \frac{1}{2} \cdot \log_x(x) \Rightarrow$ $5\sqrt{x} = x + 6 \Rightarrow 25x = x^2 + 12x + 36 \Rightarrow x^2-13x+36=0$

I believe you can solve that.

**scrible** · November 7th 2009, 06:54 AM

Originally Posted by Defunkt

It's far simpler than you're making it out to be:

By the definition of a logarithm, $log_x(x)=1$ .

Why? -- we know that $x^{log_x(a)} = a$ . So $x^{log_x(x)} = x$ and therefore $log_x(x)=1$ .

Using this and the fact that $log_b(a^n) = n\cdot log_b(a)$ for any $a,b,n$ , we get:

$5\sqrt{x} =x + 12log_x(x^{\frac{1}{2}}) \Rightarrow 5\sqrt{x} = x + 12\cdot \frac{1}{2} \cdot \log_x(x) \Rightarrow$ $5\sqrt{x} = x + 6 \Rightarrow 25x = x^2 + 12x + 36 \Rightarrow x^2-13x+36=0$

I belive you can solve that.

Thanks a million. You know I keep going over the log laws, it is the application of the laws that is killing me. Do you know of any web sight with tricky examples like these?

**Defunkt** · November 7th 2009, 07:01 AM

Originally Posted by scrible

Thanks a million. You know I keep going over the log laws, it is the application of the laws that is killing me. Do you know of any web sight with tricky examples like these?

I don't, sorry, but you should try looking for some on google.

Thread: log Problem

LinkBack

Thread Tools

Display

log Problem

Search Tags