- #1
cmkluza
- 118
- 1
Homework Statement
The function ##f## is defined on the domain ##x≥0## by ##f(x) = \frac{x^2}{e^x}##.
(a) Find the maximum value of ##f(x)##, and justify that it is a maximum.
(b) Find the ##x## coordinates of the points of inflexion on the graph of ##f##.
I believe that I did (a) correctly, but (b) is where I got stuck.
Homework Equations
Point of Inflexion Criteria:
- Is 0 at ##f''(x)##
- Changes Sign at ##f''(x) = 0##
The Attempt at a Solution
(a)
##f(x) = \frac{x^2}{e^x}##
##f'(x) = \frac{2xe^x - x^2e^x}{e^{2x}}##
∴ ##2xe^x - x^2e^x = 0##
##2xe^x = x^2e^x##
##2x = x^2##
##x=2##
Since the exponentially growing ##e^x## will increase at a much greater pace than the ##x^2## term, in ##f(x) = \frac{x^2}{e^x}##, there can be no other maxima.
(b)
This is the part I had trouble on. I figured the second derivative to be:
##f''(x) = \frac{3x^2-4x+2}{e^x}##
∴ ##3x^2-4x+2 = 0##
##x = \frac{4±\sqrt{40}}{6} \rightarrow x ≥ 0##
∴ ##x = \frac{4+\sqrt{40}}{6}##
I've probably made some mistake somewhere in my algebra, or more likely that annoying differentiation, but when I plug values on either side of that resulting x back into ##f''(x)##, there is no change in sign. Am I missing something? Have I made a mistake somewhere in my algebra or calculus? I don't believe there could be any other points that equal zero in the equation I've generated, so where'd I go wrong?
Thanks to anyone who can help me with this problem!