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March 05, 2021, 12:14:59 am

Author Topic: log question, posssibly considered ambigous?  (Read 1429 times)  Share 

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almostatrap

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log question, posssibly considered ambigous?
« on: November 06, 2009, 12:00:34 pm »
so x=-1 is not valid. and 2ln(x) does not always = ln(x^2)

I just went through the year 11 and 12 essentials textbooks, and nowhere does it mention exceptions to log laws. Even searching the internet i couldn't find it. And i have never been taught it (my teacher is awesome).

So we're just meant to know intuitively that log laws are not valid in this situation?

I declare question 9 ambiguous! marks for everyone!

I doubt this will be the case, but yeah.

what do people think?
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TrueTears

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Re: log question, posssibly considered ambigous?
« Reply #1 on: November 06, 2009, 12:01:58 pm »
I said the domain is the intersection of the two functions.

For y = 2ln(x) the domain is x > 0

For y = ln(x+3) the domain is x>-3

Thus the domain is x>0

Since -1<0

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simpak

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Re: log question, posssibly considered ambigous?
« Reply #2 on: November 06, 2009, 12:02:41 pm »
I assumed that the negative answer I got couldn't be right because then you would be taking the log of a negative.
BUT, the negative answer I got was not -1 because I made a silly mistake.
Still, same logic.
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nala

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Re: log question, posssibly considered ambigous?
« Reply #3 on: November 06, 2009, 12:06:13 pm »
I figured that rather than putting the 2 above the x, say you divided the other logs by 2 instead. You are left with loge(-1), therefore it is undefined.

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Re: log question, posssibly considered ambigous?
« Reply #4 on: November 06, 2009, 12:07:28 pm »
You have to see whether the solutions are defined in the original unaltered expression.
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almostatrap

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Re: log question, posssibly considered ambigous?
« Reply #5 on: November 06, 2009, 12:08:20 pm »
all good points! oh well
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TrueTears

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Re: log question, posssibly considered ambigous?
« Reply #6 on: November 06, 2009, 12:09:33 pm »
so x=-1 is not valid. and 2ln(x) does not always = ln(x^2)

I just went through the year 11 and 12 essentials textbooks, and nowhere does it mention exceptions to log laws. Even searching the internet i couldn't find it. And i have never been taught it (my teacher is awesome).

So we're just meant to know intuitively that log laws are not valid in this situation?

I declare question 9 ambiguous! marks for everyone!

I doubt this will be the case, but yeah.

what do people think?
Also remember squaring a number leads to redundant solutions.





However if you had you'd only be left with the positive answer .

Same logic applies, squaring the x leads to the redundant solution of x = 1.
« Last Edit: November 06, 2009, 01:36:25 pm by TrueTears »
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NE2000

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Re: log question, posssibly considered ambigous?
« Reply #7 on: November 06, 2009, 12:38:54 pm »
It's a good habit to define domains (at least in your head) before you attempt a question.
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lilyrose

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Re: log question, posssibly considered ambigous?
« Reply #8 on: November 06, 2009, 01:10:30 pm »
do you get marks off if you forgot to simplfy things?
also, i forgot to include units for rate of change and i accidently left the -ve value forlog...is that -1 mark each??

NE2000

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Re: log question, posssibly considered ambigous?
« Reply #9 on: November 06, 2009, 01:11:40 pm »
In spesh the assessment reports say that you must simplify. But I haven't seen that in a methods one so maybe not simplifying it completely will not be penalized :)

The units for rate of change, the question specifically asked you to put units so not sure. And the -ve value for log is an incorrect answer
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tomygun_123

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Re: log question, posssibly considered ambigous?
« Reply #10 on: November 06, 2009, 01:20:25 pm »
you can only take x>3/2 since it wasnt an absolute function, also taking the square of 1 will give you +- answers therefore you must reject the negative... i would think that this was just common sense since bodmas clearly applies within the brackets. But i guess it could have been a bit ambiguous.
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lilyrose

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Re: log question, posssibly considered ambigous?
« Reply #11 on: November 06, 2009, 01:25:36 pm »
In spesh the assessment reports say that you must simplify. But I haven't seen that in a methods one so maybe not simplifying it completely will not be penalized :)

The units for rate of change, the question specifically asked you to put units so not sure. And the -ve value for log is an incorrect answer

well i accidently forgot to disregard the -ve.. i still had the 3/2.. would i get one mark off for including that x=-1?

so annoyed, i made sososososo many stupid mistakes