А.
Пошаговое объяснение:
5/12-5/28=5*7/12*7-5*3/28*3=35-15/84=20/84=5/21
А. 5/21.
1) 12 = 2•2•3;
28 = 2•2•7;
НОК (12, 28) = 2•2•3•7 = 4•21 = 84.
2) 5/12 - 5/28 = 35/84 - 15/84 = 20/84 = 5/21.
