QUESTION

CSAT

Hard

Maths

Prelims 2025

Let both pp and kk be prime numbers such that (p2+k)(p^2 + k) is also a prime number less than 30. What is the number of possible values of kk?

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Explanation

We need distinct prime values of k for which there exists a prime p with p2+k<30p^2 + k < 30 and p2+kp^2 + k prime.

Since p2+k<30p^2 + k < 30 and k2k \ge 2, we have p227p{2,3,5}p^2 \le 27 \Rightarrow p \in \{2, 3, 5\}.

• Case p=2p = 2: p2+k=4+k<30k{2,3,5,7,11,13,17,19,23}p^2 + k = 4 + k < 30 \Rightarrow k \in \{2,3,5,7,11,13,17,19,23\}. Check primality of 4+k4 + k: 6 (×), 7 (✓), 9 (×), 11 (✓), 15 (×), 17 (✓), 21 (×), 23 (✓), 27 (×). Valid k: {3,7,13,19}\{3, 7, 13, 19\}.

• Case p=3p = 3: p2+k=9+k<30k{2,3,5,7,11,13,17,19}p^2 + k = 9 + k < 30 \Rightarrow k \in \{2,3,5,7,11,13,17,19\}. 9+k9 + k is prime only when k=2k = 2 (gives 11); for all odd k, 9+k9 + k is even > 2 (not prime). Valid k: {2}\{2\}.

• Case p=5p = 5: p2+k=25+k<30k{2,3}p^2 + k = 25 + k < 30 \Rightarrow k \in \{2,3\}. 27, 28 → neither is prime. Valid k: \emptyset.

Distinct k across all cases: {2,3,7,13,19}5\{2, 3, 7, 13, 19\} \rightarrow 5 values.

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