Ответ:

Пошаговое объяснение:

4/7 = (8+2a)/21

7*(8+2a) = 21*4

56+14a = 84

14a = 84 - 56

14a = 28

a = 28:14

a = 2

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1/9 = (5a-14)/54

9*(5a-14) = 54*1

45a - 126 = 54

45a = 54+126

45a = 180

a = 180:45

a = 4