Advanced Math practice question

Answer: 4

The vertex is at x = −(−4)/2 = 2, where y = 4 − 8 + c = c − 4. For the vertex to be on the x-axis, y = 0, so c = 4. (Then y = (x − 2)².)

Common wrong answers

If you answered 2

2 is the x-coordinate of the vertex, not c.

If you answered 16

16 is 4² — the discriminant condition is (−4)² − 4c = 0, which gives c = 4, not 16.

If you answered -4

−4 is the coefficient of x, or c − 4 with c = 0. The condition is c − 4 = 0.

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