Sharing The Domain?

Algebra Level 1

We learnt that log ( x 2 ) = 2 log x \log ( x^2 ) = 2 \log x .

Is the domain of f ( x ) = 2 log x f\left( x \right) =2\log { { x } } and f ( x ) = log ( x 2 ) f\left( x \right) =\log ( { x }^{ 2 } ) the same?

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2 solutions

Hung Woei Neoh
Apr 15, 2016

A simpler description:

We know that we can only take the log \log of positive numbers x > 0 x>0 . Therefore:

The domain of f ( x ) = 2 log x f(x) = 2 \log x is 0 < x < 0<x<\infty or ( 0 , ) (0,\infty) , whichever notation you prefer.

Now, remember that x 2 0 x^2 \geq 0 for all real values of x x

This means that besides x = 0 x=0 , any other value of x x will give us a positive value for x 2 x^2 , whose log \log can be calculated. Therefore:

The domain of f ( x ) = log ( x 2 ) f(x) = \log(x^2) is < x < , x 0 -\infty<x<\infty, x \neq 0 or ( , 0 ) ( 0 , ) (-\infty,0)\cup(0,\infty)

Therefore, the domains of f ( x ) = 2 log x f(x) = 2 \log x and f ( x ) = log ( x 2 ) f(x) = \log(x^2) are not the same, and the answer is No \boxed{\text{No}}

l o g ( a b ) l o g ( a ) + l o g ( b ) , for a, b R + \displaystyle log(a \cdot b) \Rightarrow log(|a|) + log(|b|) , \text{ for a, b} \in \mathbb R_{+} Note that the condition does not hold both ways and the answer is evident.

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