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# P2 logs question that seemed easy! watch

1. The question is

ln(5x+6)=2ln(5x-6)

when i worked it out i got x=2 and x=0.6

the text book says the answer is x=2 and in brackets it says x=0.6 is not possible but i can't see why because i put 0.6 back into the equation and got ln9=ln9 but i think i'm probably just being stupid(as usual! ). Can someone explain this to me please?

2. I'm guessing it is because the function is modular.
3. (Original post by K2theJ)
The question is

ln(5x+6)=2ln(5x-6)

when i worked it out i got x=2 and x=0.6

the text book says the answer is x=2 and in brackets it says x=0.6 is not possible but i can't see why because i put 0.6 back into the equation and got ln9=ln9 but i think i'm probably just being stupid(as usual! ). Can someone explain this to me please?

Because if you put it back into the equation you'd be taking logs of negative numbers - something you don't do/worry about at A-level
4. (Original post by RichE)
Because if you put it back into the equation you'd be taking logs of negative numbers - something you don't do/worry about at A-level
Oh, so you can do it at undergrad level?
5. (Original post by Jump)
Oh, so you can do it at undergrad level?
You can make sense of a log of a negative or indeed a complex number - but log is what's called a multifunction and doesn't give single values

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