Question

The general form of the equation of a circle is x^2+y^2−4x−8y−5=0.

Answer 1

X² - 4x    + y² - 8y    = 5

Complete the square:
x² - 4x + (4) +  y² - 8y + (16)    = 5 + (4) + (16)
→  (x - 2)²    +  (y - 4)²    = 25

Center: (2,4)
radius: √25 = 5