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# AS Maths help! Logarithms watch

1. Find the exact value of x for which
log2(2x)=log2(5x+4) -3

How is this done?
2. (Original post by charlyb)
Find the exact value of x for which
log2(2x)=log2(5x+4) -3

How is this done?
log2X - log2( 5X + 4 ) = -3

now simplify the LHS into one log

then rearrange according to the rule

logpq = r <=====> q = pr
3. Thank you !!

I have simplified it to log2(2x/5x+4) = -3 but now I am stuck because I thought a negative number was not able to be turned into a log ?
4. (Original post by the bear)
log2X - log2( 5X + 4 ) = -3

now simplify the LHS into one log

then rearrange according to the rule

logpq = r <=====> q = pr
Thank you !!

I have simplified it to log2(2x/5x+4) = -3 but now I am stuck because I thought a negative number was not able to be turned into a log ?
5. (Original post by charlyb)
Thank you !!

I have simplified it to log2(2x/5x+4) = -3 but now I am stuck because I thought a negative number was not able to be turned into a log ?
have a go at using the {sub}2{/sub} method for writing the bases ( use [] instead of {} ) !

negative numbers can make perfectly good logarithms with a little patience & understanding. you cannot find the log of a negative number, neither can you use a negative number as the base.

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Updated: February 13, 2016
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