f'(x) = (x-4)e^-x

Where is this function increasing and decreasing?

I only get 4 as a critical point, and plugging in tells me that its dec from (-inf,4) and inc from (4,inf) but that's wrong. Can anyone help?

Question

f'(x) = (x-4)e^-x 

Where is this function increasing and decreasing? 

I only get 4 as a critical point, and plugging in tells me that its dec from (-inf,4) and inc from (4,inf) but that's wrong. Can anyone help?

anonymous · Answer

Is that the derivative of f?

anonymous · Answer

And also, did they give you an interval to check for?

anonymous · Answer

Yeah that's the derivative

anonymous · Answer

Original function isn't given

anonymous · Answer

Ok

anonymous · Answer

No interval given nope

anonymous · Answer

So critical points are places where the derivative is 0 or undefined

anonymous · Answer

or interval endpoints

anonymous · Answer

So just 4 because e^x is all reals for domain, right? Or am I missing something there.

anonymous · Answer

Do you have the answer? Sorry, I;m in BC and we did this a while ago

anonymous · Answer

What I'm getting wrong is the intervals where it is increasing and decreasing... I put (-inf,4) for decreasing and (4,inf) and it's incorrect.

anonymous · Answer

:-(

anonymous · Answer

So 4 is the correct critical point? Because thats the only one I can find

anonymous · Answer

Yeah it's the only critical point... So I'm lost hahah

anonymous · Answer

Do you have the solution? It would help me figure this out

anonymous · Answer

I have that there is 1 critical point at 4, no local max's, and a local min at 4. That's all I'm working with

anonymous · Answer

How do you know that thats wrong?

perl · Answer

f(x) is increasing when f ' (x) > 0 
f(x) is decreasing when f ' (x) < 0

anonymous · Answer

The program I do hw on says it's wrong and it's never incorrect

anonymous · Answer

I mean thats correct what you found

perl · Answer

note that e^-x is always positive. 
so you just have to look at the factor (x-4) 
Therefore
 f(x) is increasing when x > 4 
f(x) is decreasing when x < 4

perl · Answer

is this the function 
$$
\Large 
f ' (x) = (x-4)  \cdot e^{-x}  $$

anonymous · Answer

Do you see my mistake? Seems like we are getting the same thing Perl. dec from -inf to 4 and inc from 4 to inf.

anonymous · Answer

I'm almost 100% sure that your program is wrong

anonymous · Answer

For the following 2 questions, enter your answers in the form (-2, 3], (-I,0)U[1,2), etc. 

Is there something about this form that I'm not understanding?

anonymous · Answer

Maybe just entering it wrong

anonymous · Answer

Nvm... That's not it. I D K

perl · Answer

can you upload an image of the work , a snapshot
could be a small typo

anonymous · Answer

attachment

anonymous · Answer

hahahahahah wait

anonymous · Answer

You see my error? I'm an idiot!

anonymous · Answer

L O L

anonymous · Answer

Hahahahah who puts increasing before decreasing!

anonymous · Answer

SMOOOOTH