Example:

If a = 9i - 5k, b = 2i + 3j, what is the angle between them?

Solution:

a•b = 9 x 2 + 0 x 3 + (-5) x 0 = 18

Thus the angle between a and b is

q = arccos(a•b/ab)) = arccos(18/(10.30 x 3.61))

= 61.0 degrees

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