Question

The dot product of u with itself is 12. What is the magnitude of u?

Answer 1

Dot product is 
a*b = |a|*|b|*cosα.
In our case
u*u =|u|*|u|*cos 0
u*u =|u|*|u|
12=|u|²
|u|=√12=√(4*3)=2√3
Answer: D. 2√3