Triangle Equality
Why
The squared norm of a sum of orthogonal vectors is the sum of their squared norms.
Result
Let $(V, F)$ be an inner
product space with induced
norm $\norm{\cdot }$.
Let $x, y \in V$
be orthgonal vector.
Then
\[
\norm{x + y}^2 = \norm{x}^2 + \norm{y}^2.
\]