# Existence of the Anti-Derivative.

Given a function $f(x)$ defined on interval $[a,b]$. Prove that there must exist a function $F(x)$ with the property that $F'(x)=f(x)$ $\forall x\in (a,b)$.
With this we can prove the second fundamental theorem of calculus. We have to show that if $g(x)=\int^x_a f(x)dx$ then $g'(x)=f(x)$. Instead of the classical proof with a Riemann Sum we may do the following: Since by the first fundamental theorem of calculus, "If there exists an anti-derivative of $f(x)$ then $\int^x_a f(x)dx = F(x)-F(a)$" But by the existence of anti-derivative conjecture there MUST exist such a function thus, $g(x)=F(x)-F(a)$ but then $g'(x)=f(x)$ because $F(a)$ is a constant-function.