# Why do we integrate a function to find the area under it?

1. ### Juwane

87
Why, when finding the area by definite integral, we have to find the indefinite integral first? As I understand, to find the area of under the curve, all we need is the equation of the curve. On the other hand, the indefinite integral helps us to find the original function from its derivative. So what does this have to do with finding the area?

2. ### arildno

11,265
You don't have to. If you have, in general, infinite time at your disposal.

That is the truly beautiful insight in the fundamental theorem of calculus:

To sum up the area beneath some curve, essentially an INFINITE process, can trivially be done by finding an anti-derivative to the defining curve.

3. ### Juwane

87
One of the things that the fundamental theorem of calculus tells us that the area under a curve is equal to the area under that curve's derivative, right?

4. ### elibj123

240
No, it tells us that integration (which is defined in a way that has nothing to do with derivatives or anti-derivatives) is opposite to derivation.

Which helps us in calculating integrals more efficiently.

5. ### Juwane

87
But it is true that the area under a curve is equal to the area under that curve's derivative, right?

6. ### nicksauce

1,275
No. The area under y = 1 from x = 0 to 1 is 1. The area under the derivative is 0.

118

87
9. ### rochfor1

256
Juwane: That quote you gave was just a restatement of the Fundamental Theorem of Calculus, nicksauce already gave an excellent example of where your assertion is false.

10. ### Bohrok

867
Left/right/middle sums all take too long to calculate the area?

11. ### Landau

905
@Juwane: how do get your assertion from the wikipedia quote? If $$A_f(x)$$ denotes the area function of some function f(x), then:

wiki (i.e. the fundamental theorem) says $$A_f'(x)=f(x)$$;
you're saying that $$A_f(x)=A_{f'}(x)$$.