Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Substitution error?

  1. Jan 22, 2017 #1
    I'm learning about derivatives and on the website they put the value

    x^2 into f(x + dx) and it became
    (x + dx)^2

    Shouldn't it be (x^2 + dx^2) ?

    It's the last example


    Thanks in advance!
  2. jcsd
  3. Jan 22, 2017 #2


    User Avatar
    Science Advisor

    You're thinking of it wrong. The function f(x) = x^2 says, "take the input value and square it". So f(x+dx) means "take (x+dx) and square it", which is (x+dx)^2.
  4. Jan 22, 2017 #3
    That makes sense thank you
  5. Jan 22, 2017 #4


    Staff: Mentor

    They aren't "putting the value x^2 into f(x + dx)" -- they are saying that ##f(x) = x^2## and are then calculating ##f(x + dx)## (or more likely, ##f(x + \Delta x)## ). As phyzguy noted, the given function squares its input value, so ##f(x + \Delta x) = (x + \Delta x)^2 \ne x^2 + (\Delta x)^2##.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Substitution error?