According to wiki, linkage disequilibrium $D$ equals
$$D = x_{11} - p_1\cdot q_1$$
where:
$$ \begin{matrix} \text{Haplotype} & \text{Frequency}\\ A_1B_1 & x_{11}\\ A_{1}B_{2} & x_{12} \\ A_{2}B_{1} & x_{21} \\ A_{2}B_{2} & x_{22} \\ \end{matrix} $$
and
$$ \begin{matrix} \text{Allele} & \text{Frequency}\\ A_{1} & p_{1}=x_{11}+x_{12} \\ A_{2} & p_{2}=x_{21}+x_{22} \\ B_{1} & q_{1}=x_{11}+x_{21} \\ B_{2} & q_{2}=x_{12}+x_{22} \\ \end{matrix} $$
According to Hartl and Clark, linkage disequilibrium $D$ equals:
$$D = x_{11}x_{22}-x_{12}x_{21}$$
Question
Can you please prove that these two formulations of linkage disequilibrium are equivalent (assuming that they are equivalent)? If they're not… Do we use different definitions? What are their respective meanings?