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# C2 Logarithms Watch

1. When is it a valid move to put log in front of every term in the equation? do you have to simplify the terms first?
2. I'd always simplify as far down as you possibly can (say, dividing through by a particular number). Always take care with your laws of indices from C1 as to what you can and can't do with them.

Once you're happy your equation is as simple as it could be, and your numbers could not be cancelled down further, then you should take logs.

Log both sides, not just each individual term: take the logarithm of the entire LHS and then the entire RHS.

Is there a particular question you're struggling with?
3. (Original post by oinkk)
I'd always simplify as far down as you possibly can (say, dividing through by a particular number). Always take care with your laws of indices from C1 as to what you can and can't do with them.

Once you're happy your equation is as simple as it could be, and your numbers could not be cancelled down further, then you should take logs.

Log both sides, not just each individual term: take the logarithm of the entire LHS and then the entire RHS.

Is there a particular question you're struggling with?
Ok thank you, are there any particular rules I could remember to be things that aren't allowed?
4. (Original post by Mackiemcmasher)
When is it a valid move to put log in front of every term in the equation? do you have to simplify the terms first?
If you rearrange the terms following a^x = b and x = log(base a)b, that could help. Do you have an example for your question? Then I might be able to help a bit more
5. (Original post by Mackiemcmasher)
Ok thank you, are there any particular rules I could remember to be things that aren't allowed?
Just your basic rules really, revisit C1 indices to get an idea.

One common example of something you can do to simplify your answer is:

This is a common result gained when you are finding the value of n for a particular term in a geometric series (also C2 knowledge), given its value c.

You can then take logs of both sides and use the power rule to simplify.

As I say, reduce your equation as far as you can before you take logs. It'll genuinely simplify your manipulation later. And when you have to prove a certain result, simplifying fully then taking logs is often the only way.
6. (Original post by oinkk)
I'd always simplify as far down as you possibly can (say, dividing through by a particular number). Always take care with your laws of indices from C1 as to what you can and can't do with them.

Once you're happy your equation is as simple as it could be, and your numbers could not be cancelled down further, then you should take logs.

Log both sides, not just each individual term: take the logarithm of the entire LHS and then the entire RHS.

Is there a particular question you're struggling with?
The second question on this video confuses me greatly .

http://www.examsolutions.net/maths-r...examples-1.php
7. (Original post by Mackiemcmasher)
The second question on this video confuses me greatly .

http://www.examsolutions.net/maths-r...examples-1.php
See attached my working for that one, with some notes to the side. Often, when you get to the point where 'x=', then you can type all of that into a calculator. This will not give you an exact value.

If it asks for a particular form, or an exact value, then some extra log manipulation may be required.

Which parts of the working do you not understand?

With logs, there's only a few things you can do:
• Simplify as much as you can (if you use the geometric sum or value formulae, you will often be able to simplify).
• Adding logs (to the same base, and with the same number in front of the log) means use the multiplication rule
• Subtracting logs (same rules as above) means use the division rule.
• Power rule (raise a number in front of the log to be a power, or lower the power in a log to the number in front of the log).
• Sometimes leave it as exact values, sometimes evaluate it to around 4 significant figures.
• Sometimes you may be asked to leave as an inequality, which just follows standard laws of inequalities.
8. (Original post by Mackiemcmasher)
When is it a valid move to put log in front of every term in the equation? do you have to simplify the terms first?
I remember a geometric series question where it gave numbers for the second and fourth term so to get rid of the power i had to use the rule logx^n=nlogx then divide both sides by log x to find n. Also in some part of a geometric series question my working was like 63 <360^n so again take logs of both sides bring the n down and divide by the logs. I'll let you know if i find any more. 😊
9. (Original post by oinkk)
See attached my working for that one, with some notes to the side. Often, when you get to the point where 'x=', then you can type all of that into a calculator. This will not give you an exact value.

If it asks for a particular form, or an exact value, then some extra log manipulation may be required.

Which parts of the working do you not understand?

With logs, there's only a few things you can do:
• Simplify as much as you can (if you use the geometric sum or value formulae, you will often be able to simplify).
• Adding logs (to the same base, and with the same number in front of the log) means use the multiplication rule
• Subtracting logs (same rules as above) means use the division rule.
• Power rule (raise a number in front of the log to be a power, or lower the power in a log to the number in front of the log).
• Sometimes leave it as exact values, sometimes evaluate it to around 4 significant figures.
• Sometimes you may be asked to leave as an inequality, which just follows standard laws of inequalities.
Thanks you very much! this helps a lot.
10. (Original post by Mackiemcmasher)
Thanks you very much! this helps a lot.

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