<s>[INST] If $\cos2\theta=0$, then $\Delta^2=$? I'll state the question from my textbook here:

If $\cos2\theta=0$, then $\begin{vmatrix} 0 & \cos \theta & \sin \theta \\ \cos \theta & \sin \theta & 0 \\ \sin \theta & 0 & \cos \theta \end{vmatrix}^2=$?

This is how I solved the problem:
$\begin{vmatrix} 0 & \cos \theta & \sin \theta \\ \cos \theta & \sin \theta & 0 \\ \sin \theta & 0 & \cos \theta \end{vmatrix}^2$
$= (\cos^3 \theta + \sin^3 \theta)^2$
$= (\cos \theta + \sin \theta)^2(\cos^2 \theta - \cos \theta \sin \theta + \sin^2 \theta)^2$
$= (1+ \sin2\theta)(1-\sin2\theta + \sin^2 \theta \cos^2 \theta)$
$= (1+ \sin2\theta)(1-\sin2\theta) + (1 + \sin2\theta)\sin^2\theta \cos^2 \theta$
$= \cos^2 2\theta + \frac 14 (1 + \sin2\theta)\sin^2 2\theta$
$= \frac 14 (1 + \sin2\theta)\sin^2 2\theta$
Now since $\cos2\theta=0$, $\sin2\theta = \pm 1$.
Therefore the above expression can take the values 0 and $\frac12$.
My textbook gives the answer as $\frac12$. I don't see any grounds on rejecting the other answer of 0. Have I made a mistake somewhere? Or am I forgetting something?
 [/INST] If we compute the square of the matrix, we get
$$
\begin{bmatrix}
1 & \sin\theta\cos\theta & \sin\theta\cos\theta \\
\sin\theta\cos\theta & 1 & \sin\theta\cos\theta \\
\sin\theta\cos\theta & \sin\theta\cos\theta & 1
\end{bmatrix}=
\frac{1}{2}
\begin{bmatrix}
2 & \sin2\theta & \sin2\theta \\
\sin2\theta & 2 & \sin2\theta \\
\sin2\theta & \sin2\theta & 2
\end{bmatrix}
$$
whose determinant is
$$
\frac{1}{8}(8+2\sin^32\theta-6\sin^22\theta)
$$
From $\cos2\theta=0$, we have $\sin^22\theta=1$, so finally we get
$$
\Delta^2=\frac{1}{4}(1+\sin2\theta)=
\begin{cases}
1/2 & \text{if $\sin2\theta=1$}\\[4px]
0   & \text{if $\sin2\theta=-1$}
\end{cases}
$$
both possibilities being allowed by the hypothesis that $\cos2\theta=0$.
</s>