x Turn on thread page Beta
 You are Here: Home >< Maths

# C2 Logarithm watch

1. Can anyone help me do part a for question 8 and 9? I don't even know where to start the question. I have just started doing logs and I don't know much about it! Any good tips would be useful as well!!
2. log a x = log a 5 + 2 log a 3

use laws of logs to bring the 3 up to the power

log a x = log a 5 + log a 3^2

Then use laws of logs to put the logs together

log a x = log a (5*3^2)
log a x = log a 5*9
log a x = log a 45

so x=45
for part 8 a
3. For part A of Q9, you can do the same once you've expanded the brackets, you can use the same laws, but remember that log a x - log a y = log a (x/y)
4. In question 9 you need to use 2 laws of logs - the one where "log(a) - log (b) = log(a/b), and the one where "n log (a) = log (a^n).

So you can simplify the bit in the brackets to 2 log (k/2), then you can move the 2 to being the power using the second rule so it is now log (k/2)^2. This can be expended out to log ((k^2)/4).

Comparing the equations now you know that x = (k^2)/4, so that can be rearranged to give 4x = k^2.

I hope that's clear and the notation isn't too confusing. Being familiar with the log laws is the most useful thing you can do so make sure you know them
5. (Original post by LRJ)
log a x = log a 5 + 2 log a 3

use laws of logs to bring the 3 up to the power

log a x = log a 5 + log a 3^2

Then use laws of logs to put the logs together

log a x = log a (5*3^2)
log a x = log a 5*9
log a x = log a 45

so x=45
for part 8 a
Thanks that's the correct answer!! but how do you 'bring the 3 up to the power'? Can you please tell me which law you used?

Also at the end (log a x = log a 45) did you just cancel the logs?

Thanks once again
Thanks that's the correct answer!! but how do you 'bring the 3 up to the power'? Can you please tell me which law you used?

Also at the end (log a x = log a 45) did you just cancel the logs?

Thanks once again
No prob - it's a bit late to do maths isn't it? xD

The law of log you need to bring the 3 up to the power is:
k log a x = log a x^k

And at the end, (log a x = log a 45), it's not cancelling logs, but just that A can only be 45 since the rest of the equation is exactly the same on both sides

It's kinda like equating the coefficients, i.e if Ax + B = 3x + 5
A must be 3, and B must be 5
7. (Original post by LRJ)
No prob - it's a bit late to do maths isn't it? xD

The law of log you need to bring the 3 up to the power is:
k log a x = log a x^k

And at the end, (log a x = log a 45), it's not cancelling logs, but just that A can only be 45 since the rest of the equation is exactly the same on both sides

It's kinda like equating the coefficients, i.e if Ax + B = 3x + 5
A must be 3, and B must be 5
Right I get it now!! hehe yea it is kinda late but got tonnes of homework to do so have to stay awake and finish them all xx
8. here is bear™'s non-logarithmic method:

9. (Original post by the bear)
here is bear™'s non-logarithmic method:
It’s a 5 year old thread
10. (Original post by Notnek)
It’s a 5 year old thread
ikr.. i always check my work carefully
11. (Original post by the bear)
ikr.. i always check my work carefully
Thank you for replying....5 years later. I managed to pass A level maths
Thank you for replying....5 years later. I managed to pass A level maths
http://www.2oceansvibe.com/wp-conten.../very-nice.jpg

well done sweetie

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 20, 2018
Today on TSR

### How do I turn down a guy in a club?

What should I do?

Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams