Consider the following statements: S1: There exists infinite sets A, B, C such…

2001

Consider the following statements:

S1: There exists infinite sets A, B, C such that A ∩ (B ∪ C) is finite.
S2: There exists two irrational numbers x and y such that (x+y) is rational.

Which of the following is true about S1 and S2?

  1. A.

    Only S1 is correct

  2. B.

    Only S2 is correct

  3. C.

    Both S1 and S2 are correct

  4. D.

    None of S1 and S2 is correct

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Correct answer: C

Analysis of Statement S1:

“There exists infinite sets A, B, C such that A ∩ (B ∪ C) is finite.”

To prove an existential statement ("there exists"), we only need to construct one working scenario. Let's define three infinite sets:

  • Let A = {x | x is a negative integer} = {-1, -2, -3, -4, .....}

  • Let B = {x | x is a positive even integer} = {2, 4, 6, 8, .......}

  • Let C = {x | x is a positive odd integer} = {1, 3, 5, 7, .......}

All three sets (A, B, and C) are completely infinite.

Now let's compute the operations:

  1. First, find (B C): This combines all positive evens and odds, giving us the set of all positive integers: {1, 2, 3, 4, ........}.

  2. Next, find A ∩ (B ∪ C): We look for elements common to the negative integers (A) and the positive integers (B C). There are none.

A (B C) = ϕ

The empty set (emptyset) has a size of 0, which is finite. Since we found a case where the result is finite, S1 is correct.

Analysis of Statement S2:

“There exists two irrational numbers x and y such that (x+y) is rational.”

An irrational number is a number that cannot be written as a simple fraction. Let's pick two well-known irrational numbers designed to cancel out their irrational parts when added together:

  • Let x = 2 + 3 (This is irrational)

  • Let y = 2 - 3 (This is also irrational)

Now, let's find their sum (x + y):

(2 + √3) + (2 - √3) = 2 + 2 + √3 - √3 = 4

The number 4 can be written as 4/1, which means it is a rational number.

Since we successfully found a pair of irrational numbers whose sum is rational, S2 is correct.

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