The radius of a circle = the diameter of the circle divided by 2
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Standard equation for a circle centred at the origin is x2 + y2 = r2 where r is the radius of the circle. If you increase the size of the circle then the radius must increase, so r2 will be larger. eg a circle of radius 2 has the equation x2 + y2 = 4, if the radius increases to 3 then the equation becomes x2 + y2 = 9
the equation for the circumference of a circle is 2*radius*pi, or since the radius is half of the diameter 2*radius=diameter, we can simplify the equation to the circumference of a circle=diameter*pi
Radius = 1111
x^2+y^2=36
Equation of a circle centre the origin is: x2 + y2 = radius2 ⇒ radius = √9 = 3.