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Let $S = \{(m, n) : m, n \in \{1, 2, 3, \dots, 50\}\}$. If the number of elements $(m, n)$ in S such that $6^m + 9^n$ is a multiple of 5 is p and the number of elements $(m, n)$ in S such that $m + n$ is a square of a prime number is q, then $p + q$ is equal to ____________.

To find the required sum, \( p+q \), we need to evaluate two conditions for elements \( (m,n) \) in the set \( S = \{ (m,n) : m,n \in \{1, 2, 3, \dots, 50\} \} \).

Step 1: Calculate \( p \), the number of elements where \( 6^m + 9^n \equiv 0 \pmod{5} \).

Since \( 6 \equiv 1 \pmod{5} \) and \( 9 \equiv 4 \pmod{5} \), we have:

  • \(6^m \equiv 1^m \equiv 1 \pmod{5}\).
  • \(9^n \equiv 4^n \pmod{5}\) follows the cycle \( (4,1) \), repeating every 2 terms.

We need \( 1 + 4^n \equiv 0 \pmod{5} \rightarrow 4^n \equiv -1 \equiv 4 \pmod{5} \).

This holds when \( n \) is odd since the cycle of \( 4^n \) is: \(4, 1, 4, 1, \ldots\).

Thus, for 25 odd values of \( n \), \( m \) can be any of the 50 values from 1 to 50.

Therefore, \( p = 50 \times 25 = 1250 \).

Step 2: Calculate \( q \), the number of elements where \( m+n \) is a square of a prime number.

  • Squares of prime numbers ≤ 100 (max sum of \( m, n \)): \( 2^2=4 \), \( 3^2=9 \), \( 5^2=25 \), \( 7^2=49 \).

Count solutions for each square:

SquareSolutions
43 (pairs: (1,3), (2,2), (3,1))
98 (pairs: (1,8), (2,7), ..., (8,1))
2524 (pairs from (1,24) to (24,1))
4948 (pairs from (1,48) to (48,1))

Thus, \( q = 3 + 8 + 24 + 48 = 83 \).

Final Calculation:

\( p + q = 1250 + 83 = 1333 \).

This falls within the given range [1333,1333]. Hence, the final value is \( \boxed{1333} \).

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  3. The number of $3 \times 2$ matrices A, which can be formed using the elements of the set $\{-2, -1, 0, 1, 2\}$ such that the sum of all the diagonal elements of $A^T A$ is 5, is ______
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