Chapter 1 - Section 1.9 - Distance and Midpoint Formulas; Circles - Concept and Vocabulary Check - Page 279: 4

$(x-h)^2+(y-k)^2=r^2$

RECALL: A circle with center at $(h, k)$ and a radius of $r$ units has a standard equation of the form $(x-h)^2+(y-k)^2=r^2.$ Thus, the missing expression in the given statement is $(x-h)^2+(y-k)^2=r^2.$