2-3 1/7 * (2-1 9/11) : 8/21

2 - 3 1 / 7 * (2 - 1 9 / 11) : 8 / 21 = 2 - 3 целых 1 / 7 * (2 - 1 целая 9 / 11) : 8 / 21 = 2 - (3 * 7 + 1) / 7 * (2 - (1 * 11 + 9) / 11) / (8 / 21) = 2 - (21 + 1) / 7 * (2 - (11 + 9) / 11) / (8 / 21) = 2 - 22 / 7 * (2 - 20 / 11) / (8 / 21) = 2 - 22 / 7 * (2 * 11 - 20 * 1) / 11 / (8 / 21) = 2 - 22 / 7 * (22 - 20) / 11 / (8 / 21) = 2 - 22 / 7 * (2 / 11) / (8 / 21) = 2 - 2 / 7 * 2 / (8 / 21) = 2 - 4 / 7 / (8 / 21) = 2 - 4 / 7 * (21 / 8) = 2 - 1 / 1 * (3 / 2) = 2 - 3 / 2 = (2 * 2 - 3 * 1) / 2 = (4 - 3) / 2 = 1 / 2 = 0, 5