Precalculus, Chapter 6, 6.4, Section 6.4, Problem 31

You need to use the formula of dot product to find the angle between two vectors, u = u_x*i + u_y*j, v = v_x*i + v_y*j , such that:
u*v = |u|*|v|*cos(theta)
The angle between the vectors u and v is theta.
cos theta = (u*v)/(|u|*|v|)
First, you need to evaluate the product of the vectors u and v, such that:
u*v = u_x*v_x + u_y*v_y
u*v = 1*0 + 0*(-2)
u*v = 0
Hence, since u*v = 0 , it does not matter what are the values of the magnitudes |u| and |v| since cos theta = 0 .
cos theta = 0 => theta = pi/2
Hence, the angle between the vectors u and v is pi/2 , so, the vector u is perpendicular to the vector v.