### Instruction: If we express $x^2 - 5x$ in the form $a(x - h)^2 + k$, then what is $k$?### Response: To solve this problem, I want to complete the square of $x^2 - 5x$.
That means I need to add a constant term to make it a perfect square trinomial.
To find the constant term, I can take half of the coefficient of x, which is -5, and square it.
That gives me $(-5/2)^2 = 25/4$.
So I can add and subtract 25/4 to $x^2 - 5x$, and then factor the first three terms as a perfect square.
That gives me $x^2 - 5x + 25/4 - 25/4 = (x - 5/2)^2 - 25/4$.