(1) 360
(2) 900
(3) $180(n-1)$
51. Given, $A B / / C D$.
(1) As shown in Figure (1), $\angle 1+\angle 2+\angle 3=$ $\qquad$ $\circ$;
(2) As shown in Figure (2), $\angle 1+\angle 2+\angle 3+\angle 4+\angle 5+\angle 6=$ $\qquad$ ${ }^{\circ}$;
(3) As shown in Figure (3), $\angle 1+\angle 2+\ldots+\angle n=$ $\qquad$ ${ }^{\circ}$.