It's a simple question, for a function $f(x)$, if it can be write as:

$f(x) = \frac{a}{2} + \sum_{n=1}^{\infty}a_n\sin nx + b_n\cos nx$

from this formula, we can know the energy at frequency $\frac{n}{2\pi} $ it equal to $\sqrt{a_n^2+b_n^2}$.

but for a function $f(t)$. think about its forier transform: $g(\theta)=\int f(t)e^{-i\theta t}dt$

why $g(\theta)$ tell us energy of $f(t)$ at frequency $\frac{\theta}{2\pi}$.

thanks