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Maths Alevel Mechanics, Cosine Rule, Bearings, Boats Journey Problem? - sail boat diagram

A boat travels at a speed of 11 knots to the east. The current flows in a direction of 210 ° at a speed of 6 knots. What is the resultant velocity of the boat.

With a chart with the cosine law to solve this problem, it is fairly simple. .

http://i166.photobucket.com/albums/u94/j ...

That is my plan, but would for some reason used the book for cos30 and cos60. WHY ?!?!?!?!

Please give us a solution - and an explanation (preferably with a description of a picture or a drawing one) that I will explain why using cos30 that could replace cos60.

Best answer: 5 stars.