Consider the following statements: 1. If $x$ and $y$ are composite numbers, then $x + y$ is always composite. 2. There does not exist a natural number which is neither prime nor composite. Which of the above statements is/are correct?
Aptitude
Number System
Difficulty: Medium
Choose an option
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A1 only
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B2 only
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CBoth 1 and 2
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DNeither 1 nor 2
Answer
Correct Answer: Neither 1 nor 2
Explanation
### Concept & Logic
This problem tests your theoretical understanding of number classifications.
To disprove an "always" statement in mathematics, you only need to find a single counter-example. To evaluate existence statements, you must recall boundary cases like the number $1$.
### Step-by-Step Solution
Let's evaluate each statement individually.
**Statement 1 Evaluation:**
* Claim: If $x$ and $y$ are composite, $x + y$ is always composite.
* Let's find a counter-example.
* Let $x = 4$ (a composite number).
* Let $y = 9$ (a composite number).
* Calculate the sum: $4 + 9 = 13$.
* The sum $13$ is a prime number, not a composite number.
* Because we found a case where the sum is prime, Statement 1 is false.
**Statement 2 Evaluation:**
* Claim: There does not exist a natural number which is neither prime nor composite.
* The natural numbers begin at $1, 2, 3, 4, ...$
* By definition, prime numbers have exactly two divisors. Composite numbers have more than two divisors.
* The number $1$ has exactly one divisor. Therefore, $1$ is classified as neither prime nor composite.
* Because $1$ exists, Statement 2 is false.
### Exam Strategy & Shortcut
For Statement 1, quickly mentally add small composite numbers ($4, 6, 8, 9, 10$) together until you hit a prime. $4 + 9 = 13$ is the fastest trigger. For Statement 2, always remember the unique status of the number $1$ when dealing with primes.
### Common Pitfall
Students often evaluate Statement 1 by only adding even composite numbers (e.g., $4 + 6 = 10$, $8 + 4 = 12$). Since the sum of two even numbers is always even (and therefore composite if $>2$), they incorrectly assume the statement is true. Always test a mix of even and odd composites.
### Final Answer
Therefore, the correct answer is Neither 1 nor 2.