In the pyramid $P - ABCD$, $PD \perp$ base $ABCD$, $CD \parallel AB$, $AD = DC = CB = 1$, $AB = 2$. (1) Prove that $BD \perp PA$; (2) Find the sine of the angle between $PD$ and plane $PAB$.
In the pyramid $P - ABCD$, $PD \perp$ base $ABCD$, $CD \parallel AB$, $AD = DC = CB = 1$, $AB = 2$.\\
(1) Prove that $BD \perp PA$;\\
(2) Find the sine of the angle between $PD$ and plane $PAB$.