## Circumference or circle? This is the problem!

Published on Jun 08, 2016

Circumference, circle... sometimes we use these terms thinking about the same object. But the truth is that objects are very different! Think of the difference between the wheel of a bike and a frisbee... Discover how to recognize a circle or a circle.

The circumference is the set of points that are all at a same distance from a fixed point called the center. Each segment that joins the center to a point on the circumference is called the radius. The circle, however, is the set of points that are inside a circle (if we want to put it more mathematical: the circle is the part of the plane bounded by a circle).

We know along with the elements of the circumference:

The circle also has its own parts:

Now that we have learned to recognize if a figure is a circle or a circle, let's try to draw a circle. We can draw infinite circles passing through a point, or two points, but only one through three points not aligned. Learn to recognize the points internal, external, or belonging to the circumference.

We have studied the mutual positions of straight line and circumference: a straight line may be the external secant, or tangent to the circumference. Remember the properties of tangents: all the straight lines tangent to a circle are perpendicular to a radius. It will be useful to solve many exercises!

If we are going to draw more circles, we check each other's positions: by looking at the distances between the two centers, we can understand if the circles are internal, external, tangent or secants.

Speaking of girth, we can not overlook the corners. Within a circle we can represent the two types of angles:

An angle at the centre is a large double with respect to the angle at the circumference that insists on the same arc.

Curious to learn more? See the lessons on Circumference, or circle, Circumference, points and straight lines, Angles at the centre and angles at the circumference. Watch the video lessons and workout with the exercises!

Are online video lessons and exercises explained in Arithmetic and Algebra, Geometry and Data, and the forecasts of the first, second and third year of mathematics at the middle School!
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