The graph of the equation 4x^2 + 20x + 4y^2 - 24y + 21 = 0 is a circle. Find the radius of the circle.
thank you
+37157 4x^2 + 20x + 4y^2 - 24y + 21
Divide thrugh by 4
x^2 +5x +y^2-6y = -21/4 ' complete the square ' for x and y
(x+2.5)^2 + (y-3)^2 = -21/4 + 6.25 + 9
= -21/4 + 25/4 + 36/4 = 40/4 = 10
10 = r^2 r = sqrt 10
