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# Need help on calculating the distance between position vectors watch

1. (vectors i and j are horizontal vectors in the direction due east and due north)
Two boats A and B are moving with constant velocities. Boat A moves with velocity 9j km/h. Boat B moves with velocity (3i+5j)km/h.

At noon, A is at the point O, and B is 10km due west of O. At time t hours after noon, the position vectors of A and B relative to O are a km and b km respectively.
b)Find expressions for a and b in terms of t
Ship a=6ti
Ship b=3ti + (-10+5t)j

c) find the time when B is due south of A
1400

At time t hours after noon, the distance between A and B is dkm. By finding the expression for ->AB
d) show that d^2=25t^2-60t+100

I'd appreciate some help on part d, I've tried using Pythagoras on the position vectors that I calculated in part b for ship A and B but didn't get the right answer (so the hypotenuse was the distance between AB)
2. They've got the answer for d by taking away A from B, I don't understand why you'd take away :/
3. Ignore the thread, realised the silly mistake I did

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Updated: April 9, 2013
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