“The Fundamental Theorem of Calculus” is a big name, and it sounds like it’s a very important theorem. It is. Let’s state it first, and then talk about why it’s so important.

**Definition: **Let be a continuous function on , and let be an antiderivative of *f*. Then:

Why is this so important? Well, it means that we can use antiderivatives to calculate a definite integral. Recall the limit definition of a definite integral:

It is definitely easier to take the antiderivative than to evaluate this limit of a sum. The fundamental theorem of calculus saves us a lot of work!

**Example**: Find the area under from $latex *x*=0 \text{ to } *x*=1$.

This time, we can use antiderivatives. Note that . Then, using the fundamental theorem of calculus, we know that

Note that the +*C*s cancel out. While it doesn’t matter which antiderivative we pick, it DOES matter that we pick the same antiderivative for *F*(*b*) as we do for *F*(*a*). Therefore, we usually pick *C*=0 when evaluating definite integrals.

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