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If \((a-b)^2 + (b - c)^2 + (c-a)^2 = 6\) and \(a^2 + b^2+c^2 = 29\), then what is \((a + b + c)\) equal to?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
\(\pm 9\)

Solving for \((a + b + c)\) with Given Algebraic Conditions

The problem asks us to find the value of \((a + b + c)\) given two conditions involving variables \(a\), \(b\), and \(c\). The conditions are:

  • Condition 1: \(\left(a-b\right)^2 + \left(b - c\right)^2 + \left(c-a\right)^2 = 6\)
  • Condition 2: \(a^2 + b^2+c^2 = 29\)

Expanding the First Condition

Let's start by expanding the terms in the first condition using the algebraic identity \((x-y)^2 = x^2 - 2xy + y^2\):

  • \(\left(a-b\right)^2 = a^2 - 2ab + b^2\)
  • \(\left(b-c\right)^2 = b^2 - 2bc + c^2\)
  • \(\left(c-a\right)^2 = c^2 - 2ca + a^2\)

Now, sum these expanded terms:

\(\left(a^2 - 2ab + b^2\right) + \left(b^2 - 2bc + c^2\right) + \left(c^2 - 2ca + a^2\right) = 6\)

Combine like terms:

\(2a^2 + 2b^2 + 2c^2 - 2ab - 2bc - 2ca = 6\)

Factor out the common factor of 2:

\(2\left(a^2 + b^2 + c^2\right) - 2\left(ab + bc + ca\right) = 6\)

Using the Second Condition

We are given that \(a^2 + b^2 + c^2 = 29\). Substitute this value into the equation derived above:

\(2(29) - 2\left(ab + bc + ca\right) = 6\)

\(58 - 2\left(ab + bc + ca\right) = 6\)

Now, solve for the term \(\left(ab + bc + ca\right)\):

\(58 - 6 = 2\left(ab + bc + ca\right)\)

\(52 = 2\left(ab + bc + ca\right)\)

Divide both sides by 2:

\(ab + bc + ca = \frac{52}{2}\)

\(ab + bc + ca = 26\)

Finding \((a + b + c)\)

We know the algebraic identity for the square of a sum:

\(\left(a + b + c\right)^2 = a^2 + b^2 + c^2 + 2\left(ab + bc + ca\right)\)

Substitute the known values of \(a^2 + b^2 + c^2 = 29\) and \(ab + bc + ca = 26\) into this identity:

\(\left(a + b + c\right)^2 = 29 + 2(26)\)

\(\left(a + b + c\right)^2 = 29 + 52\)

\(\left(a + b + c\right)^2 = 81\)

To find \(\left(a + b + c\right)\), take the square root of both sides:

\(a + b + c = \pm \sqrt{81}\)

\(a + b + c = \pm 9\)

Conclusion

Based on the given conditions and algebraic manipulations, the value of \((a + b + c)\) is \(\pm 9\).

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