1) (1 - 3/4) * (4/5-3/5) = (4/4-3/4) = 1/4 (4/5-3/5) = 1/5 = 1/4*1/5 = 1/20
2) 14* (1/2 +1/3+1/4) = (1/2+1/3+1/4)= общ.знам - 12 значит: 14* (6/12+4/12+3/12) = 13/12= 1 1/12 = 14 * 1 1/12 = 91/6 = 15 1/6
3) 1/3 * 6/7 - 1/7 * 4/8= 2/7 - 4/56 = 12/56 = 3/14