Для сравнения дробей, приведем их к общему знаменателю:
1) 5/21 и 3/14:
* НОК(21, 14) = 42
* 5/21 = (5 * 2) / (21 * 2) = 10/42
* 3/14 = (3 * 3) / (14 * 3) = 9/42
* 10/42 > 9/42 => 5/21 > 3/14
2) 5/16 и 3/8:
* НОК(16, 8) = 16
* 5/16 = 5/16
* 3/8 = (3 * 2) / (8 * 2) = 6/16
* 5/16 < 6/16 => 5/16 < 3/8
3) 5/10 и 1/4:
* НОК(10, 4) = 20
* 5/10 = (5 * 2) / (10 * 2) = 10/20
* 1/4 = (1 * 5) / (4 * 5) = 5/20
* 10/20 > 5/20 => 5/10 > 1/4
**Ответ:**
1) 5/21 > 3/14
2) 5/16 < 3/8
3) 5/10 > 1/4