The radius of a circle connects its center to its circumference.
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The line from the center of the circle to the oustide of the circle.
circle diameter minus center of pitch cirlce raius
A chord.
Multiply the diameter by (pi), or the radius by (2 pi). If you have a drawing of the circle, and the center is marked, you can measure the radius and the diameter. Just lay your ruler across the circle, and make sure that it touches the center. If the center isn't marked, then it won't be that easy.
Area of a circle = pi*radius2