А) (19/21+1/7)- 9/21 б ) 17/30 - ( 1/3+730)

Выполним действия сначала в скобках, потом без скобок:

А) (19/21 + 1/7) - 9/21 = 13/21;

1) 19/21 + 1/7 = 19/21 + (1 * 3)/(7 * 3) = 19/21 + 3/21 = (19 + 3)/21 = 22/21 = 1 1/21;

2) 1 1 21 - 9/21 = 22/21 - 9/21 = (22 - 9)/21 = 13/21.

Б) 17/30 - (1/3 + 7/30) = 0;

1) 1/3 + 7/30 = (1 * 10)/(3 * 10) + 7/30 = 10/30 + 7/30 = (10 + 7)/30 = 17/30;

2) 17/30 - 17/30 = (17 - 17)/30 = 0/30 = 0.

Ответ: А) 13/21. Б) 0.