Shear area,
$A_{\text{shear}} = t (w \csc \theta)$
$A_{\text{shear}} = tw \csc \theta$
$A_{\text{shear}} = A \, \csc \theta$
Shear force,
$V = P \cos \theta$
$V = \tau A_{\text{shear}}$
$P \cos \theta = \tau(A \csc \theta)$
$\tau = \dfrac{P \sin \theta \, \cos \theta}{A}$
$\tau = \dfrac{P (2 \sin \theta \, \cos \theta)}{2A}$
$\tau = \dfrac{P \sin 2\theta}{2A}$ (okay!)