а) (25/28) - (18/35) = (625/700) - (504/700) = (121/700)

б) (40/63) - (35/72) = (640/1008) - (490/1008) = (150/1008) = (25/168)

в) (22/21) - (21/22) = (484/462) - (441/462) = (43/462) = (1/14)