What is the equation of a circle with centre (2,4)

Remember that the equation of a circle is (x - h)^2 + (y - k)^2 = r^2, where (h, k) is the center and r is the radius.

Inserting the information we know, we find that the equation is:
(x - 2)^2 + (y - 4)^2 = r^2

Since, we do not have r, this equation is our final answer.