Answer
Theorem 7.2 is proved by Theorem 7.1.
Work Step by Step
We should prove theorem 7.2 by theorem 7.1.
Theorem 7.1 is the dot product of two vectors:
\[
|U| \cdot|V| \cos \theta=U \cdot V
\]
Substitute the angle in Theorem 7.1
\[
\begin{array}{l}
|U| \cdot|V| \cos 90^{\circ}=U \cdot V \\
=|U| \cdot|V| \cdot 0 \\
{[0=U \cdot V]}
\end{array}
\]
QED
