Вычислите: 1/2-1/4, 1/7*1/42, 3/18+1/2

Вычислим значения выражений: 

1) 1/2 - 1/4 = (1 * 2  - 1 * 1)/4 = (2 - 1)/4 = 1/4; 

2) 1/7 * 1/42 = 1/(7 * 42) = 1/(7 * 40 + 7 * 2) = 1/(280 + 14) = 1/294; 

3) 3/18 + 1/2 = (3 * 1 + 1 * 9)/18 = (3 + 9)/18 = 12/18 = (2 * 6)/(2 * 9) = (1 * 6)/(1 * 9) = 6/9 = (2 * 3)/(3 * 3) = (2 * 1)/(1 * 3) = 2/3.