А) 19/21+1/7-9/21= б)17/30-1/3+7/30=

А) 19/21 + 1/7 - 9/21 = 19/21 + (1 * 3) / (7 * 3) - 9/21 = 19/21 + 3/21 - 9/21 = 22/21 - 9/21 = 13/21.
Ответ: 13/21;
б) 17/30 - 1/3 + 7/30 = 17/30 - (1 * 10) / (3 * 10) + 7/30 = 17/30 - 10/30 + 7/30 = 7/30 + 7/30 = 14/30 = (14 : 2)/(30 : 2) = = 7/15.
Ответ: 7/15.