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NDA 2026 · PYQ · Quadratic Equations · hard

If the highest degree coefficient is equal to 1, then what is the total number of quadratic equations which are unchanged on squaring their roots?

Answer

The correct answer is B: 4. Let roots be α, β. After squaring, roots are α², β².

  1. A.6
  2. B.4✓ Correct
  3. C.2
  4. D.None

Explanation

Let roots be α, β. After squaring, roots are α², β². For unchanged equation, {α, β} = {α², β²}. Cases: (i) α = α², β = β² gives α, β ∈ {0, 1}, yielding pairs (0,0), (0,1), (1,1) — three equations: x², x(x−1), (x−1)². (ii) α = β², β = α² with α ≠ α²: α = α⁴ gives α³ = 1, α ≠ 1, so α is a primitive cube root of unity. This gives one equation x² + x + 1 = 0. Total = 3 + 1 = 4.
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