—It was great! Everybody enjoyed it.
将1至1997的自然数,分成A、B、C三组:
A组:1,6,7,12,13,18,19,…
B组:2,5,8,11,14,17,20,…
C组:3,4,9,10,15,16,21,…
则
若a﹣b=1,ab=﹣2,则(a+1)(b﹣1)的值为( )