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Missing root

Assuming I have the following equation:

x^2 = 9

this can easily be solved by taking the square root, to give +/- 3

Suppose instead I choose to use logs.

ln (x^2) = ln 9

2 ln x = ln 9

ln x = 0.5 ln 9

ln x = ln (9^0.5)

We know we cannot take logs of negative numbers, so all we get is:

ln x = ln 3

so x = 3

How can we get the negative root using this method?
Got r00t bro?
Reply 2
Original post by powwer
Assuming I have the following equation:

x^2 = 9

this can easily be solved by taking the square root, to give +/- 3

Suppose instead I choose to use logs.

ln (x^2) = ln 9

2 ln x = ln 9

ln x = 0.5 ln 9

ln x = ln (9^0.5)

We know we cannot take logs of negative numbers, so all we get is:

ln x = ln 3

so x = 3

How can we get the negative root using this method?

Since lnx\ln x is only defined for x>0x>0, lnx2=2lnx\ln x^2 = 2\ln x should be rewritten as

lnx2=2lnx\ln x^2 = 2\ln |x|

so that x can be both positive and negative.
(edited 9 years ago)
Original post by powwer
...


That was convoluted. If you want to lose a solution you could just do this:

x2=9x^2=9

x2=9\sqrt{x^2}=\sqrt 9

x=3x=3

If you want to solve the quadratic properly then I'm sure you know how.
Reply 4
Original post by notnek
Since lnx\ln x is only defined for x>0x>0, lnx2=2lnx\ln x^2 = 2\ln x should be rewritten as

lnx2=2lnx\ln x^2 = 2\ln |x|

so that x can be both positive and negative.


Does that mean whenever you see ln(x)\ln (x), there's always going to be two possible answers?
Reply 5
Original post by powwer
Does that mean whenever you see ln(x)\ln (x), there's always going to be two possible answers?

The only time you'd see lnx\ln x with x being negative is if a mistake has been made in the working where x accepted negative values but lnx\ln x has been used instead of lnx\ln |x|.

Assuming no mistakes, lnx=a\ln x = a has only one solution for x, namely x=eax=e^a.

It probably won't affect you in any exam or textbook question but a good idea is to always check what x could be before taking logs. Similarly, always check that x is not 0 before dividing by it.

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