# Indefinate integral of e

1. Mar 25, 2008

### DeepSeeded

I was under the impression that the derivative of e raised to any power with a variable in it is just the same result. So why isnt it the same for all indefinate integrals of e to any power with a variable in it? Specificaly why is it that:

\int e^6x = \frac{1}{6} e^6x

Last edited: Mar 25, 2008
2. Mar 25, 2008

### Vid

I'm assuming you mean e^(6x).

It's a simple u-sub. u = 6x. du = 6dx du/6 = dx

Then Integral[e^(6x)dx] becomes Integral[e^(u)*dx/6] = 1/6*e^(u) = 1/6*e^(6x)

3. Mar 25, 2008

### DeepSeeded

Ok that makes sense. However the integral is an anti-derivative. This means that now taking the derivative of the answer:

F = 1/6*e^(6x)
F' = e^(6x)

Is that true?

I thought it was F' = 1/6*e^(6x)

4. Mar 25, 2008

### sutupidmath

yeah, when you take the derivative of the primitive function F(x) it should equal the integrand. that is

$$\int f(x)=F(x)+C=>[F(x)+C]'=f(x)$$

Integrals and derivatives, in some sense are inverse to each other, that is one undoes what the other does.

5. Mar 25, 2008

### DeepSeeded

Ok I was wrong about e, e raised to a single variable with no coefficient only comes out to be the same answer.
Otherwise it comes out to be the product of the coefficient and the same answer. Thats what threw me off to begin with. Thanks

Last edited: Mar 25, 2008
6. Mar 26, 2008

### Mute

In general you have to use the chain rule:

$$\frac{d}{dx}e^{f(x)} = e^{f(x)}\frac{d f(x)}{dx}$$

When f(x) = x you get the result you were originally familiar with.

7. Mar 26, 2008

### HallsofIvy

Staff Emeritus
This is incorrect. The derivative of ex is ex. If the exponent just "has a variable in it", i.e. is a function of x, they you must use the chain rule:
$$\frac{de^{f(x)}}{dx}= e^{f(x)}\frac{df}{dx}$$

8. Mar 26, 2008

### tectactoe

In this case, $$\int e^{6x}dx = \frac{1}{6}e^{6x} + C$$

To check, take the derivative of $$\frac{1}{6}e^{6x} + C$$ and you should come back to $$e^{6x}$$.

The $$\frac{1}{6}$$ is pulled through the derivative sign by constant multiple rule and the C drops out, so you're dealing with

$$\frac{d}{dx}(e^{6x})$$ which follows the rule $$\frac{d}{dx}(e^{u}) = e^{u}\frac{du}{dx}$$

and $$\frac{d}{dx}(6x) = 6$$

So, $$\frac{d}{dx}(e^{6x})$$ gives you $$6e^{6x}$$

and don't forget about the constant 1/6 you pulled out earlier...

$$(\frac{1}{6})(6e^{6x}) = e^{6x}$$

It checks out.