=[20 13×(20 13- 1)]/[20 12×(20 12+ 1)

=20 13×20 12/20 12×20 13

= 1

b = 20 14x 20 14-20 14/20 13x 20 13+20 13

=20 14×(20 14- 1)/20 13×(20 13+ 1)

= 1

c = 20 15x 20 15-20 15/20 14x 20 14+20 14

=20 15×(20 15- 1)/20 14×(20 14+ 1)

= 1

∴abc= 1× 1× 1= 1