b = 6.300000000000001 (slider from 3 to 25, step 0.1)
kk: 1 / 2 / b
tf = 3600
sf = 0.28
dt = 0.9000000000000001 (slider from 0.3 to 2, step 0.1)
l = 8.3 (slider from 1 to 30, step 0.1)
k = 0.045454545454545456
f = Function(tf x sf / (l + x sf dt + kk (x sf)^(2)), 0, 250)
g = Function(Derivative[f], 0, 250)
R = Root(g, 20)
E: (x(R), f(x(R)))
a = Line(R, E)
f1 = Function(tf / (l / (x sf)), 0, 80)
f2 = Function(tf / (dt + x sf kk), 0, 250)