1. Find the common divisors of the following arrays, and thereby find their greatest common divisor:
(i) $72, -60$;
(ii) $-120, 28$;
(iii) $168, -180, 495$.
1. (i) $\mathscr{D}(72,-60)=\{ \pm 1, \pm 2, \pm 3, \pm 4, \pm 6, \pm 12\},(72,-60)=12$;
(ii) $\mathscr{T}(-120,28)=\{ \pm 1, \pm 2, \pm 4\},(-120,28)=4$;
(iii) $\mathscr{D}(168,-180,495)=\{ \pm 1, \pm 3\},(168,-180,495)=3$.