i.e., z is a vector orthogonal to the vector v (Indeed, z is the projection of u onto the plane orthogonal to v.). We can thus apply the Pythagorean theorem to
which gives
and, after multiplication by left| v
ight|^2, the Cauchy–Schwarz inequality. Moreover, if the relation '≥' in the above expression is actually an equality, then left| z
ight|^2 = 0 and hence z = 0; the definition of z then establishes a relation of linear dependence between u and v. This establishes the theorem.