Р = 12 3/5 + (12 3/5 - 2 3/30) + х = 28 13/20
12 3/5 - 2 3/30 = 63/5 - 63/30=378/30 - 63/30 = 378 - 63/30 = 315/30 = 10 15/30 = 10 1/2 вторая сторона
28 13/20 - (12 3/5 + 10 1/2) = 573/20 - (63/5 + 21/2) = 573/20 - (126 +105/10) = 573/20 - 231/10 = 573/20 - 462/20 = 573-426/20 = 111/20 = 5 11/20 третья сторона