Title: Krzysztof's Example Deck

Front: [latex]
Give a lower bound on $\left| a \right|$.
[/latex]
Back: [latex]
We know that
\[
\left| a \right|\ge 0.
\]
[/latex]

Front: [latex]
Let $\left\Vert \cdot \right\Vert $ be some norm. Why can we say
\[
\left\Vert a + b \right\Vert \le \left\Vert a \right\Vert + \left\Vert b \right\Vert ?
\]
[/latex]
Back: [latex]
This is the triangle inequality, one of the defining properties of norms.
[/latex]