Let f(x)=15/(1+4e^(-0.2x) )

What is the point of maximum growth rate for the logistic function f(x)? Show all work.

Round your answer to the nearest hundredth

6.93

Step-by-step explanation:

f(x) = 15 / (1 + 4e^(-0.2x))

f(x) = 15 (1 + 4e^(-0.2x))^-1

Taking first derivative:

f'(x) = -15 (1 + 4e^(-0.2x))^-2 (-0.8e^(-0.2x))

f'(x) = 12 (1 + 4e^(-0.2x))^-2 e^(-0.2x)

f'(x) = 12 (1 + 4e^(-0.2x))^-2 (e^(0.1x))^-2

f'(x) = 12 (e^(0.1x) + 4e^(-0.1x))^-2

Taking second derivative:

f"(x) = -24 (e^(0.1x) + 4e^(-0.1x))^-3 (0.1e^(0.1x) − 0.4e^(-0.1x))

Set to 0 and solve:

0 = -24 (e^(0.1x) + 4e^(-0.1x))^-3 (0.1e^(0.1x) − 0.4e^(-0.1x))

0 = 0.1e^(0.1x) − 0.4e^(-0.1x)

0.1e^(0.1x) = 0.4e^(-0.1x)

e^(0.1x) = 4e^(-0.1x)

e^(0.2x) = 4

0.2x = ln 4

x = 5 ln 4

x ≈ 6.93

Graph: desmos.com/calculator/zwf4afzmav