Solution
= Solution
A real two-dimensional <antisymmetric matrix> has the form $A=\begin{pmatrix}0&s\\-s&0\end{pmatrix}$. Its <determinant> is $s^2=1/4$, and the specified positive upper-right entry selects $s=1/2$. Thus
$$
\boxed{A=\begin{pmatrix}0&1/2\\-1/2&0\end{pmatrix}}.
$$