.Какая из данных сумм наименьшая?

(13/18+1/7)+5/18
(5/16+1/14)+11/16
2/5+(1/18+3/5)
3/17+(1/14+14/17)

1. Сначала решим все выражения.

1)

(13/18 + 1/7) + 5/18 = (91/126 + 18/126) + 35/126 = 109/126 + 35/126 = 144/126 = 1 1/7

2)

(5/16 + 1/14) + 11/16 = (70/224 + 16/224) + 154/224 = 86/224 + 154/224 = 240/224 = 1 1/14

3)

2/5 + (1/18 + 3/5) = 36/90 + (5/90 + 54/90) = 36/90 + 59/90 = 95/90 = 1 1/18

4)

3/17 + (1/14 + 14/17) = 42/238 + (17/238 + 196/238) = 42/238 + 213/238 = 255/238 = 1 1/14

2. Теперь приводим к общему знаменателю.

1 1/7 = 8/7 = 16/14 = 288/252

1 1/14 = 15/14 = 270/252

1 1/18 = 19/18 = 266/252

1 1/14 = 15/14 = 270/252

Ответ: 2/5+(1/18+3/5)