a) 4/5 - 3/4 = (4 * 4)/(5 * 4) - (3 * 5)/(4 * 5) = 16/20 - 15/20 = (16 - 15)/20 = 1/20
в) 2/3 - 1/7 = (2 * 7)/(3 * 7) - (1 * 3)/(7 * 3) = 14/21 - 3/21 = (14 - 3)/21 = 11/21
д) 5/6 - 5/12 = (5 * 2)/(6 * 2) - 5/12 = 10/12 - 5/12 = (10 - 5)/12 = 5/12
Ответ: а) 1/20; в) 11/21; д) 5/12