11/12*12/11 = 1
13/14*14/13 = 1
и так далее
11/12*12/11*13/14*14/13*15/16*16/15*17/18*18/17*19/20 = 19/20
11/12*12/11 = 1
13/14*14/13 = 1
и так далее
11/12*12/11*13/14*14/13*15/16*16/15*17/18*18/17*19/20 = 19/20