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JEE Mains · Maths · STD 11 - 4.2 Quadratic equations and inequations

Let \(m\) and \(n\) be the numbers of real roots of the quadratic equations \(x^2-12 x+[x]+31=0\) and \(x ^2-5| x +2|-4=0\) respectively, where \([ x ]\) denotes the greatest integer \(\leq x\). Then \(m ^2+ mn + n ^2\) is equal to \(..............\).

  1. A \(9\)
  2. B \(8\)
  3. C \(7\)
  4. D \(6\)
Verified Solution

Answer & Solution

Correct Answer

(A) \(9\)

Step-by-step Solution

Detailed explanation

\(x ^2-12 x +[ x ]+31=0\) \(\{ x \}= x ^2-11 x +31\) \(0 \leq x ^2-11 x +31 < 1\) \(x ^2-11 x +30 < 0\) \(x \in(5,6)\) \(\text { so } \quad[ x ]=5\) \(x ^2-12 x +5+31=0\) \(x ^2-12 x +36=0\) \(x =6 \quad \text { but } x \in(5,6)\) \(\text { so } \quad x \in \phi\)…
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