Answer:
5
Step-by-step explanation:
The equation of a circle in standard form is
(x - h)² + (y - k)² = r²
where (h, k) are the coordinates of the centre and r is the radius
To express
x² - 8x + y² = 9 in this form complete the square on the x- terms
x² + 2(- 4) x + (- 4)² + y² = 9 + (- 4)²
(x - 4)² + y² = 9 + 16 = 25 ← in standard form
with centre = (4, 0) and r = [tex]\sqrt{25}[/tex] = 5
