1. For $x \in \mathbb{R}$ and $y \in \mathbb{R}$ define
$d_1(x, y) = (x - y)^2$,
$d_2(x, y) = \sqrt{|x - y|}$,
$d_3(x, y) = |x^2 - y^2|$,
$d_4(x, y) = |x - 2y|$,
$d_5(x, y) = \frac{|x - y|}{1 + |x - y|}$.
Determine, for each of these, whether it is a metric or not.