The circumference of a circle is its perimeter
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if a circle has an 18 meter diameter, then it has a 9 meter radius the circumference of a circle is 2(pi)r so the c. would be 2(pi)(9) which is 18pi. (another way to think of circumference is the diameter times pi)
the way you find the circumference is 2xpixr or pixdiameter. so it would be 2x80xpi
Circumference and the formula for circumference is C=πd or C=2rd
You need to know the radius first. Next you simply double the radius to get the diameter and then multiply that by pie and you have now worked out the circumference of your circle :)
Not sure what you mean by "in the middle of a circle". A chord must have each of its ends on the circumference of the circle. If a chord is such that is passes through the centre point of the circle, it is (the longest chord possible and is) called a diameter. Or to put it another way, a diameter of a circle is a chord which passes through the centre point of the circle.