Find all pairs of consecutive odd positive integers both of which are smaller than 10 such that their sum is more than 11.


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Solution:
Let the two consecutive odd positive integers be x and x + 2.
x < 10, x + 2 < 10, x + (x + 2) > 11

x + 2 < 10

x < 8

x + (x + 2) > 11

2x + 2 > 11

2x > 9
x > 9/2

i.e 8 > x > 9/2

Thus, the required pairs of consecutive odd positive integers are (5, 7) and (7, 5).

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Let x and x + 2 be the required pairs of consecutive odd positive integers. 

Given: x + 2 < 10

⇒ x < 8 and

⇒ x + x + 2 > 11

⇒ 2x > 9

⇒ x > 4.5

∴ 4.5 < x < 8 x = 5, 7

∴ Required possible pairs are (5, 7), (7, 9).