What is the domain of the function \(f(x)=\sqrt{x^2-6x+5}\)?

Correct answer: \(( -\infty,1]\cup[5,\infty)\)

For a real square root, the expression inside the root must be non-negative. Thus, \(x^2-6x+5\ge 0\). Factoring gives \((x-1)(x-5)\ge 0\), which is true for \(x\le 1\) or \(x\ge 5\). Therefore, the domain is \(( -\infty,1]\cup[5,\infty)\). Option \([1,5]\) is incorrect because the quadratic is negative between its roots. Exam tip: for any square-root function, set the radicand greater than or equal to zero first.