See attachment. It should be right but I'm wrong.

Printable View

- October 1st 2010, 12:08 PMsolidstatemathwhy is this derivative wrong?
See attachment. It should be right but I'm wrong.

- October 1st 2010, 12:13 PMSpringFan25
fine so far, but you haven't finished.

now differentiate with respect to x.

...

... - October 1st 2010, 12:18 PMSpringFan25
also, according to wikipedia, the answer can be written down directly:

Differentiation under the integral sign - Wikipedia, the free encyclopedia

But you should use whatever method your teacher is expecting.

**Edit:**Ignore this, not applicable here. (would be if the upper limit was x instead of x^3 ) - October 1st 2010, 12:46 PMsolidstatemath
Ok I understand what you did in the first four lines. I don't get the part where you say differentiate with respect to x and it goes "- 0 " at the end. Shouldn't it be -1 if the 0.25 was factored out like that?

- October 1st 2010, 12:50 PMSpringFan25
Yes on the factoring, but the derivative of 1 is 0.

**Note**There MAY be a mistake in my working somewhere else, as i dont think the answer that this will produce is consistent with what wikipedia says. However wikipedia math articles routinely have mistakes in them, so we will see ^^ - October 1st 2010, 12:59 PMsolidstatemath
ok so the - 0 at the end isn't needed leaving us with just 0.25 d/dx * e^-4x^3 and isn't that what I wrote or should I ad "dt"?

This would give (0.25 d/dx * e^-4x^3) * dt

or is it because they ask for the derivative of d/dx that I must replace "t" by "x^3"? - October 1st 2010, 01:01 PMSpringFan25
I dont see that in your screenshot. Even if it was what you meant, you haven't answered the question. you have to actually DO the differenciation.

the answer is not d/dx "times" something. The answer is the derivative of 0.25 e^(-4x^3)

**NB**

Make sure you understand what the notation means. It does not mean that there is some fraction being multuiplied by a function, f(x). - October 1st 2010, 01:06 PMsolidstatemath
I don't understand.

The way I read your line 4 is like this: 0.25 ( d/dx ( e^-4x^3 ) -0 )

When I see this I read 0.25 and then the derivative of what's inside. If that is it then it should be e^-4x^3 * -12x^2 and we would get 0.25 * ( e^-4x^3 * -12x^2 ). - October 1st 2010, 01:13 PMSpringFan25
and what does the textbook say the answer is?

- October 1st 2010, 01:19 PMsolidstatemath
There is none. The tutor I went to see gave me this to work with:

integral sign = S

S (e^ax) * dx = 1/a * e^ax

so I got: S (e^-4t)dt = -1/4 * e^-4t

Then I think I sub t for x^3 but I'm not sure. I really don't know what to do after the sub. Do I apply F(b) - F(a) because I'm really lost. I'm so used to doing the F(b)-F(a) thing that when they ask this I just end up totally lost. - October 1st 2010, 01:22 PMPlato
I think that you are missing the point of the question.

It is well known that if is continuous on and then

So with the aid of the chain rule - October 1st 2010, 01:31 PMSpringFan25
0.25 * ( e^-4x^3 * -12x^2 ) was correct.

You already did F(b) - F(a) right at the start to obtain this answer.

Starting over:

You want to find

which i will write as

From the previous discussion, you should agree that

"something" =

you get this by doing the integral in the normal way, including the F(b) - F(a) step at the end.

**NOW**start thinking about the derivative of "something", using the chain rule.

differenciate

*chain rule*

- October 3rd 2010, 01:34 PMsolidstatemath
Okay just did this again from scratch, this is what I got:

1/4 [(-e^-4x^3) + 1]

What do you guys think? - October 3rd 2010, 02:23 PMArchie Meade