Advanced Math practice question

Answer: -8 or −8

The vertex of y = x² + kx + 5 is at x = −k/2. Setting −k/2 = 4 gives k = −8.

Common wrong answers

If you answered 4

4 is the vertex's x-coordinate, not k.

If you answered 8

8 has the wrong sign; the vertex is at −k/2, so a vertex at +4 needs k = −8.

If you answered -4

−4 forgets the factor of 2: the vertex is at −k/(2 · 1), so k = −2 × 4.

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