Jair Goh's answer to Charles's Junior College 1 H2 Maths Singapore question.

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Jair Goh
Jair Goh's answer
4 answers (A Helpful Person)
1st
Since u is reduced to a unit vector in the end, it does not matter which u you use. You modulus the thing because acute angles must be positive. (0,0,1) is just a vector on the z axis