Fourier Transform of exponential While solving Example 4.1 of Signals and Systems by Alan Oppenheim. Example 4.1 is:
$$ x(t)=e^{-at}u(t), a&gt;0$$
and the transform I get is:
$$ X(j\omega)\frac{1}{a+j\omega}, a&gt;0$$

The problem is understanding the sketch: 
The sketch is drawn for both the values $+a$ and $-a$> i dont understand why it $-a$ when $a&gt;0$ is defined.
The plot is of $$\mid X\left(i\omega\right) \mid = \sqrt{\left(\frac{1}{a+j\omega}\right)\left(\frac{1}{a-j\omega}\right)} = \frac{1}{\sqrt{a^2 + \omega^2}}$$ 

against $\omega$

In particular $\omega$ can be equal to $-a$. 

This checks out with Wolfram alpha