I'm confused by this. Could someone explain?

Question

Given two vectors, a = (a1, a2, a3) and b = (b1, b2, b3) describe how you would determine whether they are perpendicular.

Answer 1

If 2 vectors are perpendicular, then their dot product = 0

a dot b = a1b1 + a2b2 + a3,b3

if that is zero, you have perpendicular vectors

e.g. u = <1,2,3> and b = < 2,5, -4>
then a dot b = 2 + 10 - 12 = 0 , so the vectors a and b are perpendicular.