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MATHS: do my logarithmic question pls (exam paper Q)

Started by antisickness, April 20, 2013, 12:01:45 PM

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antisickness

Use logarithms to solve the equation , giving the value of x correct to 3 S.F. (5 marks)

pls kthx

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antisickness


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lasercut

log both sides:
log (7^x) = log (2^x-1)

take the x and x-1 outside log:
x log7 = (x-1) log 2

multiply out:
x log 7= x log 2 - log 2

x log 7 - x log 2 = - log 2
x (log 7 - log 2) = - log 2

x = - log 2 / (log 7 - log 2) = -0.553

i winz
OMG I updated my avatar

Toast

i was joking, i don't even know what logarithms is.


Iridion

Quote from: Toast on April 20, 2013, 12:35:58 PM
i was joking, i don't even know what logarithms is.
they dont teach that in kindergarten :D


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mo9

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antisickness

Quote from: lasercut on April 20, 2013, 12:32:04 PM
log both sides:
log (7^x) = log (2^x-1)

take the x and x-1 outside log:
x log7 = (x-1) log 2

multiply out:
x log 7= x log 2 - log 2

x log 7 - x log 2 = - log 2
x (log 7 - log 2) = - log 2

x = - log 2 / (log 7 - log 2) = -0.553

i winz

seems most legit. here is my thanks priz: WINNER

Epic xD

April 20, 2013, 01:59:56 PM #10 Last Edit: April 20, 2013, 02:06:43 PM by Epic xD
lasercut is the only person who solved that question correctly...

logs are pretty ez bro.

there be three r00lz:
1) if you are adding to logs, you can combine the two logs by multiplying by the numbers "inside"
ex: log 2 + log 3 = log (2*3) = log 6

2) if you are subtracting logs, you can combine two logs by dividing by the numbers "inside"
ex: log 6 - log 3 = log (6/3) = log 2

3) if you have an exponent in a log, you can move that exponent to the front and distribute it
ex: log 6^(x-1) = (x-1) * log 6 = (x * log 6) - log 6

source: i did logs in algebra II and got a 98% in the class

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antisickness

^ got th low down br0 was jus harrrd in the brain

mo9

Quote from: antisickness on April 20, 2013, 01:32:21 PM
Quote from: lasercut on April 20, 2013, 12:32:04 PM
log both sides:
log (7^x) = log (2^x-1)

take the x and x-1 outside log:
x log7 = (x-1) log 2

multiply out:
x log 7= x log 2 - log 2

x log 7 - x log 2 = - log 2
x (log 7 - log 2) = - log 2

x = - log 2 / (log 7 - log 2) = -0.553

i winz

seems most legit. here is my thanks priz: WINNER
EPILEEEPSYYYYYY 
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vv secks box

123bomb123

IGN: MisterZ

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because.


Rub me out like genies, won't concede til I'm graffiti
Quote from: Yz on January 14, 2012, 06:48:35 PM
mrz is like top 40 and would probly be like
"fuk yeh n00bs im god bow to god''

123bomb123

IGN: MisterZ

ZeaL - Rank 1 in antagonizing, just
because.


Rub me out like genies, won't concede til I'm graffiti
Quote from: Yz on January 14, 2012, 06:48:35 PM
mrz is like top 40 and would probly be like
"fuk yeh n00bs im god bow to god''

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