Fundamental Theorem of Calculus
Derivation is the opposite of integration.
\[f(x)=\frac{d}{dx}\int_a^{x}{f(t)dt}\]
Often used with the chain rule:
\[\frac{d}{dx}\int_a^{g(x)}{f(t)dt}=g'(x)\cdot f(g(x))\]
For now I’m assuming this will be clear enough in exercises.