Question

A car and a truck leave the same intersection, the truck heading north at 60 mph and the car heading west at 55 mph. At what rate is the distance between the car and the truck changing when the car and the truck are 30 miles and 40 miles from the intersection, respectively?

Answer 1

Answer: -81

Explanation:

Let x = the car moving towards west = 30

Let y = truck moving towards north = 40

dx/dt = -55

dy/dt = -60

We the use Pythagoras theorem to solve further where,

s² = x² + y²

s² = 30² + 40²

s² = 900 + 1600

s² = 2500

s =✓2500

s = 50

We then differentiate with respect to t, then we will have:

s² = x² + y²

2s ds/dt = 2x ds/dt + 2y dy/dt

50 ds/dt = (30 × -55) + (40 × -60)

50 ds/dt = -1650 - 2400

50 ds/dt = - 4050

ds/dt = -4050/50

ds/dt = -81