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January 30, 2015

January 30, 2015

Posted by **Shawn** on Monday, December 1, 2008 at 2:34pm.

31.) log(x-4)+ log(x+4)= log4

I have the answer as 4

Is this answer right?

- Math help -
**bobpursley**, Monday, December 1, 2008 at 2:42pmlog a + log b is same as log ab

so log(x^2-4)=log4

taking antilogs

x^2-4=4 solve for 4

check my thinking.

- Math help -
**Shawn**, Monday, December 1, 2008 at 2:57pmOkay x^2-4 = 4

x^2= 8

sqrt(x^2)= sqrt 8

x= 2.83 is this right?

- Math help -
**Reiny**, Monday, December 1, 2008 at 3:04pmlog k has a nasty habit of becoming undefined if k is not positive.

you answer of x = 2.832 is correct according to the steps taken, but...

when we sub into log (x-4) we get log (-1.168 which is undefined,

so there is no solution in the set of real numbers for your equation

- Correction: Math help -
**Reiny**, Monday, December 1, 2008 at 3:21pmGoing back and looking over the previous solutions, I noticed that the original non-log equation should have said:

x^2-16 = 4

so x^2 = 20

x = √20 = 4.472136

which works upon substituting.

- Correction: Math help -

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