Math Problem Statement

Given positive integers a and b, find all such primes p that satisfy

a(b^2 - p) + b(a^2 - p) = 5p

Solution

To solve this equation, we need to analyze the relationship given for prime pp in terms of the integers aa and bb. The equation provided is:

a(b2p)+b(a2p)=5pa(b^2 - p) + b(a^2 - p) = 5p

Step-by-Step Solution

  1. Expand and Rearrange: Start by expanding the terms in the equation:

    ab2ap+ba2bp=5pab^2 - ap + ba^2 - bp = 5p

    Combine like terms involving pp on one side and constants on the other:

    ab2+ba2=5p+ap+bpab^2 + ba^2 = 5p + ap + bp

  2. Factor Out pp: Factor pp on the right side of the equation:

    ab2+ba2=p(5+a+b)ab^2 + ba^2 = p(5 + a + b)

    Thus, we have:

    p=ab2+ba25+a+bp = \frac{ab^2 + ba^2}{5 + a + b}

  3. Constraints on pp: Since pp is a prime number, ab2+ba25+a+b\frac{ab^2 + ba^2}{5 + a + b} must result in an integer. This condition restricts aa and bb to values that make the numerator a multiple of 5+a+b5 + a + b.

  4. Testing Small Values for aa and bb: Let's analyze small integer values for aa and bb and check if the result yields a prime pp.

    • Case a=1a = 1, b=1b = 1: p=112+1125+1+1=1+17=27(not an integer)p = \frac{1 \cdot 1^2 + 1 \cdot 1^2}{5 + 1 + 1} = \frac{1 + 1}{7} = \frac{2}{7} \quad \text{(not an integer)}

    • Case a=1a = 1, b=2b = 2: p=122+2125+1+2=4+28=68=34(not an integer)p = \frac{1 \cdot 2^2 + 2 \cdot 1^2}{5 + 1 + 2} = \frac{4 + 2}{8} = \frac{6}{8} = \frac{3}{4} \quad \text{(not an integer)}

    • Case a=2a = 2, b=3b = 3: p=232+3225+2+3=29+3410=18+1210=3010=3p = \frac{2 \cdot 3^2 + 3 \cdot 2^2}{5 + 2 + 3} = \frac{2 \cdot 9 + 3 \cdot 4}{10} = \frac{18 + 12}{10} = \frac{30}{10} = 3

      Here, p=3p = 3, which is a prime number.

    • Case a=3a = 3, b=2b = 2: p=322+2325+3+2=34+2910=12+1810=3010=3p = \frac{3 \cdot 2^2 + 2 \cdot 3^2}{5 + 3 + 2} = \frac{3 \cdot 4 + 2 \cdot 9}{10} = \frac{12 + 18}{10} = \frac{30}{10} = 3

      Again, p=3p = 3, which is a prime number.

Conclusion

For the cases (a,b)=(2,3)(a, b) = (2, 3) and (a,b)=(3,2)(a, b) = (3, 2), the equation holds with p=3p = 3, which is a prime.

Thus, the solution for pp is: p=3\boxed{p = 3}

Would you like more detailed steps or have any questions?

Here are some related questions you might explore:

  1. What would happen if we set aa and bb as non-integer values?
  2. Can we generalize this approach to larger primes?
  3. How would the equation change if we replaced 5 with another prime number?
  4. Could there be other integer pairs (a,b)(a, b) for different primes pp?
  5. What if we impose additional constraints on aa and bb, like parity or divisibility?

Tip: When testing equations for integer or prime solutions, try small values first to spot patterns or feasible solutions quickly.

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Math Problem Analysis

Mathematical Concepts

Algebra
Prime Numbers
Diophantine Equations

Formulas

a(b^2 - p) + b(a^2 - p) = 5p
p = (ab^2 + ba^2) / (5 + a + b)

Theorems

Properties of Prime Numbers
Diophantine Equation Analysis

Suitable Grade Level

Grades 11-12