Wow, a question I actually know! The radius of a circle is 1/2 of the circumfrence. In other words, if your body was a circle, and you spread out youre arms, the area from you're heart to the tip of you're fingers would be the radius.
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It means to multiply the radius by itself: radius x radius
The radius of the circle is half the diameter of the circle. 30mm radius would mean 60mm diameter or 6cm.
I'm not sure what you mean. If the radius is 5, the circumference is 10pi. (about 31 and a half) If the circumference is 5, the radius is 5 divided by 2pi. (about four fifths)
What do you mean by the radius of 4? Radius is used in circles. Do you mean that the breadth is 4? If so you can use Pythagoras's Theorem to find the 'slant height' (provided that it is a right-angle triangle) (slant height)2=52+42
2 x Pi x r is equal to the circumference of a circle whose radius is equal to r.