Ответы и объяснения

2014-02-06T15:49:32+04:00

17/30 - (1/3+7/30)= 17/30 - (11/30+7/30)=17/30 - 18/30 = -1/30

(19/21+1/7) - 9/21 = (19/21 + 3/21) - 9/21 = 22/21 - 9/21 = 13/21