The radius of a circle whose equation is x² + y² + 8x – 6y + 21 = 0 is 5 units.

We know that the general form of a circle is as follows.

(x – a)² + (y-b)² = r²

=> x² + y² – 2ax – 2by + (a² + b² – r²) = 0

And we know that radius = r = √(a² +b² )

So, comparing the given equation and the general form of the equation.

we get

=> -2ax = 8x => a = – 4

=> -2by = -6y => b = 3

So, r = √((-4)² +3² ) = √16 + 9 = √25 = 5

So, the radius r of a circle whose equation is x² + y² + 8x – 6y + 21 = 0 is 5 units.

• This reply was modified 2 years, 1 month ago by ProtonsTalk.