# Homework Help: Finding the Relationship between the Limit Method and Direct Integral Method for Area

1. Nov 30, 2011

### vanmaiden

1. The problem statement, all variables and given/known data
I am in the process of studying integration and finding the areas under curves. So far, I know of two methods of finding the area under a curve: the limit method and the direct integral method. Could someone explain the relationship between these two methods?

2. Relevant equations
$\int$f(x) dx = F(x)|$^{b}_{a}$ = F(b) - F(a) = Area

$lim_{n→∞}$ $\sum^{n}_{i = 1}$ $f(x_{i})$Δx = Area

3. The attempt at a solution
I noticed in the direct integration method for finding the area under a curve that the area under the curve is equal to the change in y of a more complicated function: the integral. I graphed it out on my calculator and I don't see exactly how this works.

$lim_{n→∞}$ $\sum^{n}_{i = 1}$ $f(x_{i})$Δx = Δy of F(x) = Area

I'm trying to seek an explanation as to why the limit method yields the same result as the direct integral method.

2. Nov 30, 2011

3. Dec 3, 2011

### vanmaiden

Re: Finding the Relationship between the Limit Method and Direct Integral Method for

Ah yes, I read through a bit of it. I'm rather confused on the proof for the First Fundamental Theorem of Calculus where it is
F(x) = $\int^{x}_{a}f(t) dt$
I've just never seen an antiderivative represented in this way before. Could you interpret this for me? Why does an antiderivative have an upper and lower bound?

Last edited: Dec 3, 2011
4. Dec 3, 2011

### LCKurtz

Re: Finding the Relationship between the Limit Method and Direct Integral Method for

Let's say you have a function f(x) and its antiderivative F(x) so you might have written $$F(x)=\int f(x)\, dx + C$$ where F'(x) = f(x). If you were going a definite integral you would write $$\int_a^b f(x)\,dx = (F(x)+C)|_a^b = F(b) - F(a)$$ and the C is usually omitted since it cancels out anyway.

Now the x in that definite integral is a dummy variable, not affecting the answer, so that line could as well have been written$$\int_a^b f(t)\,dt = F(t)|_a^b = F(b) - F(a)$$ Since this is true for any a and b, let's choose to let b be a variable x:$$\int_a^x f(t)\,dt = F(t)|_a^x = F(x) - F(a)$$ Since these are equal you still have F'(x) = f(x) so the left side is an antiderivative of f(x). Since a can be anything, the F(a) is like the constant of integration in our first equation.

5. Dec 3, 2011

### vanmaiden

Re: Finding the Relationship between the Limit Method and Direct Integral Method for

Yes!!! So, to make sure I have this correct, F(a) = C in this case, correct?

6. Dec 3, 2011

### LCKurtz

Re: Finding the Relationship between the Limit Method and Direct Integral Method for

I would leave it as F(a). Here's an example. Suppose you are trying to find the function whose derivative is x2 and whose value at x = 0 is 4. You might do it this way:$$f(x) = \int x^2\, dx =\frac {x^3}{3}+C$$ Then you plug in x = 0 to require that f(0) = 4 and that tells you that C = 4 so your answer is$$f(x) = \frac{x^3} 3+4$$ Alternatively you could have solved the problem this way:$$f(x)-f(0)=\int_0^x t^2\, dt = \frac{t^3} 3 |_0^x =\frac {x^3} 3$$ where f(0) = 4, which you could have put in in the first place. Same answer, slightly different methods.

7. Dec 3, 2011

### vanmaiden

Re: Finding the Relationship between the Limit Method and Direct Integral Method for

If f(0) = 4, then shouldn't f(x) - f(0) = $\frac{x^{3}}{3}$ - 4?

8. Dec 3, 2011

### LCKurtz

Re: Finding the Relationship between the Limit Method and Direct Integral Method for

f(x) - 4 = $\frac{x^3} 3$