Answer:
radius = 8
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
Given
x² + y² - 2x + 8y - 47 = 0
Collect the x- terms and the y- terms together and add 47 to both sides
x² - 2x + y² + 8y = 47
Complete the square on both x and y terms
add ( half the coefficient of the x/y term )² to both sides
x² + 2(- 1)x + 1 + y² + 2(4)y + 16 = 47 + 1 + 16
(x - 1)² + (y + 4)² = 64 ← in standard form
with r² = 64 ⇒ r =
= 8