Fundamental Theorem of Calculus

Why it belongs

Often cited as the most beautiful equation in mathematics.

$$
\frac{d}{dx} \left( \int_a^x f(t)\,dt \right) = f(x)
$$

\[ \frac{d}{dx} \left( \int_a^x f(t)\,dt \right) = f(x) \]

This theorem is calculus.