# Thread: Natural log, solve for x, undefined?

1. ## Natural log, solve for x, undefined?

If you solve this eq for x: $\ln (5-x^2)=4$
you get:
$x=\sqrt{5-e^4}}$

but that's not defined, is it?

Is it still a correct answer? Since obviuosly if you insert the above for x, you will get 4.

2. No, I don't think you did something wrong...

$5-x^2 = e^4 \approx 54.6$

It means that x^4 must be negative, which we all know can only be positive since the power is even. There are no real solutions.