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When we use a compass, we always need to make sure the radius of the circle with the ruler, it is troublesome and inaccuracy. Circling combines a compass and a tape measure, on the basis of the characteristics of the isosceles triangle----the connection between two midpoints on the two waist is equal to half of the length of bottom line, you can acquire the radius more accuracy and quickly. You just need to pull the arms of the compass to find the corresponding length and multiply by 2, then you will get the correct length(which can be accurate to the millimeter).
Xuan Teng Huijun Chen