1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Darivative problems! which one is wrong?

  1. Oct 4, 2009 #1
    1. The problem statement, all variables and given/known data
    http://img197.imageshack.us/img197/8162/probc.jpg [Broken]
    http://img197.imageshack.us/img197/8162/probc.jpg [Broken]

    2. Relevant equations
    in the picture

    3. The attempt at a solution
    i am not sure about the 4th and 6th ones but i am almost 100% sure that everything else is right.
    can you please explain to me which one is wrong?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 4, 2009 #2
    Two of your answers are incorrect. Question 4 is correct, but question 6 is not. Use the hint in number 6 and let x = 0. Do you see why it must be false? I leave you to find the other incorrect answer.

  4. Oct 5, 2009 #3
    oh that makes sense.. if u plug in 0, f+g will equal 0 too... which means the derivative is gonna be the same....
    i get it.........

    now i am sure that everything is good.... but the last one is tricky.. should be right though..
    f(x) the derivative is f'(x) not g(x) so that must be false.
    maybe the 3rd one? even though i read about it.. and if u plug in e^x, it should stay the same..

    please help me out here
  5. Oct 5, 2009 #4
    For the last one, it might be easier to write it out as.
    f(x) = pi x^-2

    Find the derivative, see if it matches.

    For the 3rd, try out a few other possibilities. You should point out to your teacher that when a=e, that e^x works.
  6. Oct 5, 2009 #5
    oh... that why i got it wrong because i assumed that i can plug in anything but 1.. so 'e' should have worked.. but i guess that a mistake in the system that they forgot to put that note up.

    yeah i know the derivative but i wasnt sure if thats what trips me.

    tnx for help.
  7. Oct 5, 2009 #6


    User Avatar
    Science Advisor

    Why are you "plugging" things in? Problem 3 asks about [itex]a^x[/itex], NOT [itex]e^x[/itex]. And problem 6 has nothing to do with the value at x= 0.
  8. Oct 5, 2009 #7
    i plugged in just to check if it works..
    if i plug in any number but 1, it shouldnt work. since 'e' is a number too technically, it should work.

    anyways, i solved the problem and i got the idea.. so problem solved :)
    tnx for help!
  9. Oct 6, 2009 #8


    User Avatar
    Science Advisor

    But apparently you still do not understand the problems. By putting "a= e" in problem 2 you chose the only value of a for which it is true. 2 is false.
  10. Oct 6, 2009 #9
    I believe HallsofIvy means question 3 here.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook