Question 96832
If you have vectors (2,3) and (-1,6)
Add the corresponding x and y values to get:
(1,9)
Then the angle is tan^-1(9/1) = 83.66 degrees
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Be careful when you take the tan^-1; you must 
remember tan^-1 has a restricted range.
Visualize where your resultant is so you get
the proper angle.
For example: 
If the resultant is (-3,-5), tan^-1(-5/-3) = 59.036.. degrees.
But that resultant is in the 3rd quadrant. The angle you 
want is 180+59.036 = 239.036 degrees.
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Cheers,
Stan H.