In each of the following, two open sentences $P(x, y)$ and $Q(x, y)$ are given, where the domain of both $x$ and $y$ is $\mathbf{Z}$. Determine the truth value of $P(x, y) \Leftrightarrow Q(x, y)$ for the given values of $x$ and $y$.
(a) $P(x, y): x^2-y^2=0$. and $Q(x, y): x=y$. $(x, y) \in\{(1,-1),(3,4),(5,5)\}$.
(b) $P(x, y):|x|=|y|$. and $Q(x, y): x=y$. $(x, y) \in\{(1,2),(2,-2),(6,6)\}$.
(c) $P(x, y): x^2+y^2=1$. and $Q(x, y): x+y=1$. $(x, y) \in\{(1,-1),(-3,4),(0,-1),(1,0)\}$.