中等MATH-ALGE-012
The equation x^2 − 6x + k = 0 has a repeated real root. What is k?
A. 3
B. 9✓ 答案
C. 12
D. 36
解說
A repeated root means the discriminant is zero: 36 − 4k = 0, so k = 9. The repeated root itself is 3, which is not the value of k. The value 36 is b^2, before it is set against 4k.