Posted by **lisa** on Saturday, July 9, 2011 at 9:08pm.

At noon, ship A is 30 nautical miles due west of ship B. Ship A is sailing west at 21 knots and ship B is sailing north at 15 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.)

this is a cal problem.

- math -
**Damon**, Saturday, July 9, 2011 at 9:30pm
after 4 hours

x = - 30 - 21*4 = - 114

y = 15*4 = 60

dx/dt = -21

dy/dt = 15

D at 4 hr = sqrt(114^2+60^2) = 129

D^2 = x^2 + y^2

2 D dD/dt = 2 x dx/dt + 2 y dy/dt

129 dD/dt = -114(-21) + 60(15)

dD/dt = 25.5 knots

- math -
**lisa**, Saturday, July 9, 2011 at 11:06pm
i think ur answer is wrong

- math -
**Reiny**, Sunday, July 10, 2011 at 12:21am
I got the same answer as Damon.

Damon is right.

- math -
**lisa**, Sunday, July 10, 2011 at 12:25am
i type it in the computer , its says its wrong. but thanks

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