I'm not sure what I'm doing wrong here...

S(t) = 20,000(1 + e^{-0.5t})

Find the rate of change when t=1

S'(t) = 20,000(1 + e^{-0.5t}) d/dx(-0.5t)

= -10,000(1+e^{-0.5t})

S'(1) = -10,000(1+ e^{-0.5[1]})

= -10,000(1 + (0.606530659)

= -10,000(1.606530659)

= -16,065

Apparently I should be getting -6,065 instead. I realize that this has something to do with the fact that the '1 + ...." is still in the expression, when it shouldn't be, but I'm not sure how I should be getting rid of it..