Chapter 4.6, Problem 15E

Estimating time Suppose you want to travel D miles at a constant speed of (60 + x) mi/hr, where x could be positive or negative. The time in minutes required to travel D miles is $T\left(x\right)=60D{\left(60+x\right)}^{-1}$.

  15.    Show that the linear approximation to T at the point x = 0 is $T\left(x\right)\approx L\left(x\right)=D\left(1-\frac{x}{60}\right)$.