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View Full Version : Logarithmic Functions - argh - help please

Charlie
21-09-2005, 09:05 PM
I've been trying hours to do 2 questions for my maths homework. I've got the answer but don't have a clue how to get there. Could someone point me in the right direction please?

1) The sketch below shows the graph y=log10 (x+c). Find the value of c.

http://img341.imageshack.us/img341/5314/graph13so.jpg

2) The sketch below shows the graph y=alog2 (x+b). Find a and b

http://img341.imageshack.us/img341/6838/graph24hv.jpg

I've worked out b to be 4 (which is right), I just can't get a.


Answers:
1) 3
2) a= 3 1/3 b= 4


------

Thanks a lot in advance.
Space
21-09-2005, 09:09 PM
Mr_Odwin will have a field day with this.
bob
21-09-2005, 09:09 PM
If you have 'b', then you can work out 'a' just by using two values of 'x' and 'y' from the graph.
Charlie
21-09-2005, 09:13 PM
If you have 'b', then you can work out 'a' just by using two values of 'x' and 'y' from the graph.

I've got it down to 10=alog2(8).

What do you do with the 8 in the brackets? Can you just take it away to get
2= alog2? Then a=log2/2?

:confused:
bob
21-09-2005, 09:18 PM
Is that a log to the base 2?
Dieter
21-09-2005, 09:18 PM
Sorry man, it's been a bit long since I've had logarithmic functions, I need a freshen-up :)

Oh snap, that's the next thing we revise >.<
Charlie
21-09-2005, 09:19 PM
Is that a log to the base 2?

Yeah

(You could just do all the working if you want, but I think it would probably be better for me if you just give me pointers...)
Twozzok
21-09-2005, 09:20 PM
i'm guessing this is the scottish equivalent to A Level work?

we do nothing like this at GCSE... though i'm gonna be doing it at A Level ^_^
Charlie
21-09-2005, 09:23 PM
i'm guessing this is the scottish equivalent to A Level work?

we do nothing like this at GCSE... though i'm gonna be doing it at A Level ^_^

GCSE = Standard Grade (what I did last year)

So if A level is the one after GCSE then that is what it is. Highers, they're called. And they're bloody hard.
bob
21-09-2005, 09:28 PM
Bugger knows.....we only did log to the base e.


Wait for Odwin methinks. Althoughhe did say he hated pure maths. There was a dude in the Uni thread who said he was doing maths at Uni. Forget his name though.
Charlie
21-09-2005, 09:29 PM
Bugger knows.....we only did log to the base e.


Wait for Odwin methinks. Althoughhe did say he hated pure maths. There was a dude in the Uni thread who said he was doing maths at Uni. Forget his name though.

I hope he drops by in the next 30 mins. This is meant to be in for tomorrow....
bob
21-09-2005, 09:32 PM
Dude, if you can't do it, then just tell he teacher you can't do it. They can't berate you for something you can't do. Especially if you've tried for two hours.
Charlie
21-09-2005, 09:34 PM
Dude, if you can't do it, then just tell he teacher you can't do it. They can't berate you for something you can't do. Especially if you've tried for two hours.

Yeah, I wil. At least I've actually tried the question. She just said "there better not be any blanks".
bob
21-09-2005, 09:36 PM
She sounds like a witch.
Blackfox
21-09-2005, 10:02 PM
Logs are awful. I skipped them and still passed my exams..:rolleyes:
entropy
21-09-2005, 10:25 PM
I think the log10 means it is base 10 so we have:
y = log10 (x+c)
0 = log10 (c -2) [from graph]
10^0 = c-2
1 = c-2 [anything to power of 0 is 1]
1+2 = c
therefore c = 3

log2 is log to base 2, so solve simultaneously:
1) 10 = alog2 (4+b)
2) 0 = alog2 (b-3)
we'll work with 2) first:
0/a = log2 (b-3)
0 = log2 (b-3)
2^0 = b-3
1 = b-3
1+3 = b
therefore b = 4
plug b = 4 into 1):
10 = alog2 (4+4)
10 = alog2 (8)
10 = log2 (8^a) [one of the rules, look in your txt bk]
2^10 = 8^a
1024 = 8^a
ln1024/ln8 = 10/3 = 3 1/3
bob
21-09-2005, 10:38 PM
Well entropy owned this thread.
The3rdChildren
21-09-2005, 10:49 PM
MAAAAATHS. My nemesis.
zatoichi
21-09-2005, 11:24 PM
Why don't they teach relevant stuff in maths any more? I mean, when are ya gonna use a graph like that? Certainly not buying oranges. Ooooohhh no. You just need good old fashioned arithmetic for that!
SupaFry
22-09-2005, 05:58 AM
I was once good at maths but now it just hurts my brain.
RATM_4_EVA
22-09-2005, 06:06 AM
oh god i hate math.
Lammie
22-09-2005, 06:13 AM
Man I feel really unintelligent.
Ramar
22-09-2005, 06:35 AM
Logs..mein eyes!!

Thank god I dropped Maths after AS.
Daniel
22-09-2005, 12:19 PM
pfff i start maths at uni next week and ive never used logs to the base 2 only to base ten and e

Edit: come to think of it i only did log graphs in physics..
Charlie
22-09-2005, 03:07 PM
I think the log10 means it is base 10 so we have:
y = log10 (x+c)
0 = log10 (c -2) [from graph]
10^0 = c-2
1 = c-2 [anything to power of 0 is 1]
1+2 = c
therefore c = 3

log2 is log to base 2, so solve simultaneously:
1) 10 = alog2 (4+b)
2) 0 = alog2 (b-3)
we'll work with 2) first:
0/a = log2 (b-3)
0 = log2 (b-3)
2^0 = b-3
1 = b-3
1+3 = b
therefore b = 4
plug b = 4 into 1):
10 = alog2 (4+4)
10 = alog2 (8)
10 = log2 (8^a) [one of the rules, look in your txt bk]
2^10 = 8^a
1024 = 8^a
ln1024/ln8 = 10/3 = 3 1/3

Thanks. My teacher showed me how to do it today and her way was the same. :)
zatoichi
22-09-2005, 04:35 PM
Now let's talk about how to solve the root position of the prime number of the graph of logs, while simultaneously solving pi to the nth degree of the price of an apple.
Mr_Odwin
22-09-2005, 05:11 PM
I wasn't around last night, sorry Moria, I would've happily helped.
mario114
22-09-2005, 09:58 PM
I hate maths, physics is better!
CVD
22-09-2005, 10:26 PM
Don't know why, but everytime my matchs teacher said 'right, get your logs out' I burst out laughing.

Still got a B for maths and cruising into uni.
Charlie
23-09-2005, 04:28 PM
Don't know why, but everytime my matchs teacher said 'right, get your logs out' I burst out laughing.

Still got a B for maths and cruising into uni.

Must be a Scottish thing, I started laughing just reading it. :)