Homework Statement
Prove the triangle inequality for the following metric d
d\big((x_1, x_2), (y_1, y_2)\big) = \begin{cases}
|x_2| + |y_2| + |x_1 - y_1| & \text{if } x_1 \neq y_1 \\
|x_2 - y_2| & \text{if } x_1 = y_1
\end{cases},
where x_1, x_2, y_1, y_2 \in \mathbb{R}...