Since the speed is the instantaneous rate of change of height, let us use the balanced difference quotient formula with h = 0.0001 to approximate the derivative.

speed at time t = = d'()	≈	d(+0.0001) - d(-0.0001) 2×0.0002
	=	d(.0001) - d(.9999) 0.0002
	≈	- 0.0002
	=	ft/sec,

which happens to be the exact answer!