Question

The general form of the equation of a circle is x2+y2−4x−8y−5=0.

What are the coordinates of the center of the circle?



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Answer 1

centre = (2, 4)

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 - 4x - 8y - 5 = 0

Rearrange the x/y terms together and add 5 to both sides

x² - 4x + y² - 8y = 5

Use the method of completing the square on both the x/y terms

add ( half the coefficient of the x/y terms )² to both sides

x² + 2(- 2)x + 4 + y² + 2(- 4)y + 16 = 5 + 4 + 16

(x - 2)² + (y - 4)² = 25 ← in standard form

with centre (2, 4) and r =
`√(25)` = 5