## Calculus: Early Transcendentals (2nd Edition)

In general, the definite integral $$\int_{a}^bf(x)dx$$ gives the net area of the region bounded by the graph of the function, the $x$ axis, and the vertical lines $x=a$ and $x=b$. When $a=b$ this region appears to have no width and thus its net area is equal to zero and so is the integral $$\int_a^af(x)dx.$$