Two Endpoints and One Subtraction
Figure 1 — The shaded area equals the change in the antiderivative between the endpoints. The left vertical side is , the right is , and the area between them is their difference.
If is continuous on and is any antiderivative of there, then
Three consequences are worth holding at once. The choice of does not matter, because adding a constant adds it to both terms and it cancels. The result is signed, so an interval that dips below the axis produces a smaller value than the geometric area. And the theorem requires continuity on the closed interval, which is the real constraint in practice.
| Situation | What to do | Why |
|---|---|---|
| integrand is a polynomial, sine, cosine, exponential | antidifferentiate, then subtract | the theorem applies directly |
| integrand is already | evaluate at the endpoints | the theorem collapses one level |
| integrand has a zero inside the interval | integrate piecewise if area was requested | the signed total is not the area |
| integrand has a vertical asymptote inside | stop, the theorem does not apply | the antiderivative is not continuous there |