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If \(\frac{a^2}{b^2+c^2}=\frac{b^2}{c^2+a^2}=\frac{c^2}{a^2+b^2}\), then what is the value of \(a^4+b^4+c^4\) equal to ?

This question was previously asked in
CDS 2 2026 Maths Question Paper (13-Sep-2026)
The correct answer is

\(a^2b^2+b^2c^2+c^2a^2\)

To solve the problem, we need to find the value of \( a^4 + b^4 + c^4 \) given the condition \(\frac{a^2}{b^2+c^2}=\frac{b^2}{c^2+a^2}=\frac{c^2}{a^2+b^2}\). Let's denote this common ratio as \( k \). Thus, we have the equations:

  • \(\frac{a^2}{b^2 + c^2} = k \Rightarrow a^2 = k(b^2 + c^2)\)
  • \(\frac{b^2}{c^2 + a^2} = k \Rightarrow b^2 = k(c^2 + a^2)\)
  • \(\frac{c^2}{a^2 + b^2} = k \Rightarrow c^2 = k(a^2 + b^2)\)

By summing these equations, we get:

\(a^2 + b^2 + c^2 = k[(b^2 + c^2) + (c^2 + a^2) + (a^2 + b^2)]\)

Simplifying the right side:

\(a^2 + b^2 + c^2 = k[2(a^2 + b^2 + c^2)]\)

Thus:

\(1 = 2k \Rightarrow k = \frac{1}{2}\)

Substituting back, we have:

  • \(a^2 = \frac{1}{2}(b^2 + c^2)\)
  • \(b^2 = \frac{1}{2}(c^2 + a^2)\)
  • \(c^2 = \frac{1}{2}(a^2 + b^2)\)

Additionally, adding these equations yields:

\(a^2 + b^2 + c^2 = \frac{1}{2}[2(a^2 + b^2 + c^2)] = a^2 + b^2 + c^2\)

This confirms our value of \( k \). Now, consider each of the original squared expressions:

By plugging \( a^2 = \frac{1}{2}(b^2 + c^2) \), \( b^2 = \frac{1}{2}(c^2 + a^2) \), and \( c^2 = \frac{1}{2}(a^2 + b^2) \) into \( a^4 + b^4 + c^4 \), we can show:

\(a^4 + b^4 + c^4 = \left(\frac{1}{2}(b^2 + c^2)\right)^2 + \left(\frac{1}{2}(c^2 + a^2)\right)^2 + \left(\frac{1}{2}(a^2 + b^2)\right)^2\)

By simplifying, each squared term yields:

\(\frac{1}{4}(b^4 + 2b^2c^2 + c^4), \frac{1}{4}(c^4 + 2c^2a^2 + a^4), \frac{1}{4}(a^4 + 2a^2b^2 + b^4)\)

Summing all the expanded squared terms gives:

\(\frac{1}{4}(2a^4 + 2b^4 + 2c^4 + 2(a^2b^2 + b^2c^2 + c^2a^2)) = a^2b^2 + b^2c^2 + c^2a^2\)

Thus, the value of \( a^4 + b^4 + c^4 \) is \( a^2b^2 + b^2c^2 + c^2a^2 \). Therefore, the correct option is:

Option 3: \( a^2b^2 + b^2c^2 + c^2a^2 \)

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