Cauchy-Schwarz Inequality:
If $\textbf{u}$ and $\textbf{v}$ are vectors in a real inner product space $V$, then $$|\left\langle\textbf{u},\textbf{v}\right\rangle|\leq||\textbf{u}||\ ||\textbf{v}||$$
What will happen with the Cauchy-Schwarz inequality if the angle between the two vectors is zero?