Find the derivative of f(x) = 1000 / (0.1x + 1) where 10 <= x <= 70

write it as

y = 1000(.1x + 1)^-1
dy/dx = -1000(.1x + 1)^-2 (.1)
= -100/(.1x + 1)^2

The function and its derivative are both defined for your given domain.