The radius of the circle whose equation is x² + y² + 8x - 6y + 21 = 0 is 2 units
Radius of a circle
Since the equation of the circle is x² + y² + 8x - 6y + 21 = 0, to find the radius, we complete the square in the terms x and y and convert it to the form (x - a)² + (y - b)² = r² (1) which is the standard equation of a circle.
So, x² + y² + 8x - 6y + 21 = 0
x² + 8x + y² - 6y + 21 = 0
x² + 8x + (8/2)² - (8/2)² + y² - 6y + (-6/2)² + (-6/2)² + 21 = 0
x² + 8x + 4² - 4² + y² - 6y + (-3)² - (-3)² + 21 = 0
(x + 4)² - 4² + (y - 3)² - (-3)² + 21 = 0
(x + 4)² - 16 + (y - 3)² - 9 + 21 = 0
(x + 4)² + (y - 3)² - 16 - 9 + 21 = 0
(x + 4)² + (y - 3)² - 4 = 0
(x + 4)² + (y - 3)² = 4 (2)
Comapring (1) and (2), r² = 4
So, r = √4
r = 2
So, the radius of the circle whose equation is x² + y² + 8x - 6y + 21 = 0 is 2 units
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