Expii

# What Is an Integral Function? - Expii

The simplest kind of integral function, which is relevant to FTC, is $$F(x) = \int_a^x f(t)\,dt$$, where $$f$$ is a function and $$a$$ is a constant (so only the upper endpoint, $$x$$, varies). Note that when you plug in a value of $$x$$ less than $$a$$, you get a backwards definite integral. Later on, we'll also look at more complicated integral functions.