Вычислите: 9/12 + 11/12; 15/18 - 13/18; 51/60 - 17/60

Вычислим значения выражений: 

1) 9/12 + 11/12 = (9 * 1 + 11 * 1)/12 = (9 + 11)/12 = 20/12 = (2 * 10)/(2 * 6) = (1 * 10)/(1 * 6) = 10/6 = (2 * 5)/(2 * 3) = (1 * 5)/(1 * 3) = 5/3; 

2) 15/18 - 13/18 = (15 * 1 - 13 * 1)/18 = (15 - 13)/18 = 2/18 = 2/(2 * 9) = 1/(1 * 9) = 1/9; 

3) 51/60 - 17/60 = (51 * 1 - 17 * 1)/60 = (51 - 17)/60 = (50 + 1 - 10 - 7)/60 = (40 - 6)/60 = 34/60 = (2 * 17)/(2 * 30) = (1 * 17)/(1 * 30) = 17/30.