Posted by **chelsea** on Wednesday, March 23, 2011 at 1:49pm.

Let q(x)=(x^4)(e^-x). Calculate q'(x). Use q'(x) to determine whether q(5)<q(6) or q(5)>q(6) (do not use a calculator). Explain your reasoning.

- calculus -
**MathMate**, Thursday, March 24, 2011 at 1:06pm
use the product rule to calculate

q'(x)=4x3e^(-x)+x4*(-e^(-x)

= (4-x)x^3*e^(-x)

Since e^(-x)>0 on ℝ, this means

that q'(x) has zeroes at x=0 and x=4 only.

q'(5)=-125e^(-5)<0

which demonstrates that q(x) is strictly monotonically *decreasing* on [4,∞) and therefore...

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