At noon, ship A is 40 nautical miles due west of ship B. Ship A is sailing west at 21 knots and ship B is sailing north at 22 knots. How fast (in knots) is the distance between the ships changing at 4 PM? (Note: 1 knot is a speed of 1 nautical mile per hour.)
another one?
The distance z is
z^2 = (10+21t)^2 + (22t)^2 = 925t^2 + 420t + 100
2z dz/dt = 1850t + 420
now find dz/dt at t=4