A circle with radius 5 is tangent to the x-axis at the origin. What is the equation of the circle?
Correct: C
Since the circle is tangent to the x-axis at the origin and has a radius of 5, its center must be at (0, 5) or (0, -5). Since the radius is 5, the circle must be above x-axis. Therefore, the center is at (0, 5). The equation of a circle with center (h, k) and radius r is (x-h)^2 + (y-k)^2 = r^2. In this case, (x-0)^2 + (y-5)^2 = 5^2, which simplifies to x^2 + (y-5)^2 = 25.