trig
posted by jonathan .
Two ships, one sailing at 30 km/hr and the other at 45 km/hr, left port at the same time. Three hours later they were 120 km apart. If you had to find the angle between their courses an equation that could be used to solve this problem is:

looks like a straightforward application of the cosine law.
120^2 = 90^2 + 135^2  2(90)(135)cosß
where ß is the angle between them 
Ship A went 30 * 3 = 90 km
Ship B went 45 * 3 = 135 km
So we have a triangle with legs of 90 and 135 and an opposite side of 120
Use the law of cosines
120^2 = 90^2 + 135^2  2*90*135 cos T
solve for T
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