For which values of is for every real ?

Answer and explanation

Correct answer: C.

  1. Step 1: An upward parabola is positive everywhere when it has no real roots: .
  2. Step 2: So , which gives .
Answer
.
Why not the others: or is where the discriminant is positive (so the parabola crosses the axis); uses ; includes large where there are real roots; leaves out negative such as , for which the parabola is still positive.

Practise more Conditions on a parameter.

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