Definition:Definite Integral

Let $$\left[{a \,. \, . \, b}\right]$$ be a closed interval of the set $$\mathbb{R}$$ of real numbers.

Let $$f: \mathbb{R} \to \mathbb{R}$$ be a real function.

Let $$f \left({x}\right)$$ be bounded on $$\left[{a \,. \, . \, b}\right]$$.

Suppose that $$\exists y \in \mathbb{R}$$ such that:


 * For any lower sum $$L$$ over any of subdivision of $$\left[{a \, . \, . \, b}\right]$$, $$L \le y$$;
 * For any upper sum $$U$$ over any of subdivision of $$\left[{a \, . \, . \, b}\right]$$, $$U \ge y$$.

Then $$y$$ is known as the ''definite integral of $$f \left({x}\right)$$ over $$\left[{a \,. \, . \, b}\right]$$'' and is denoted $$y = \int_a^b f \left({x}\right) dx$$.

It follows from Upper Sum Never Smaller than Lower Sum that $$\sup L = \int_a^b f \left({x}\right) dx = \inf U$$.

$$f \left({x}\right)$$ is formally defined as ''(properly) integrable over $$\left[{a \,. \, . \, b}\right]$$ in the sense of Riemann''.

More usually (and informally), we say "$$f \left({x}\right)$$ is integrable over $$\left[{a \, . \, . \, b}\right]$$".

If $$a > b$$ then we define $$\int_a^b f \left({x}\right) dx = - \int_b^a f \left({x}\right) dx$$.

Geometric Interpretation
The expression $$\int_a^b f \left({x}\right) dx$$ can be (and frequently is) interpreted as the area under the graph.

A depiction of the lower and upper sums illustrates this:



It can intuitively be seen that as the number of points in the subdivision increases, the more "accurate" the lower and upper sums become.

Also note that if the graph is below the $$x$$-axis, the area under the graph becomes negative.