If \(a\) and \(b\) are real numbers, which condition is necessary and sufficient for the equation \(x^2-2(a-2b)x+(a+2b)^2=0\) to have real roots?

Correct answer: \(ab\leq 0\)

A quadratic equation has real roots when its discriminant satisfies \(D\geq0\). Here, \(D=[-2(a-2b)]^2-4(a+2b)^2=4\{(a-2b)^2-(a+2b)^2\}=-32ab\). Therefore, \(-32ab\geq0\), which gives \(ab\leq0\). Option D is only a sufficient condition: \(a+2b=0\) implies \(ab\leq0\), but it is not necessary. Exam tip: For questions about the nature of quadratic roots, begin by calculating the discriminant and imposing \(D\geq0\).