A circle is a special kind of ellipse in which the eccentricity is zero and the two foci are coincident. The circles divide the plane into two regions such as interior and exterior regions. It is similar to the type of line segment. Imagine that the line segment is bent around till its ends join. Arrange the loop until it is exactly circular in shape.
In maths projects for class 10 on circles, the construction of a circle and all the properties and terminologies are explained in detail. To understand what circles are in simple terms, go through circles for class 10, and also try the following exercise –
 Take an empty sheet of paper and just mark a single point on the sheet, somewhere in the middle of the sheet, and name it to point O.
 Select a random length for radius, for example, 3 cm.
 Using a ruler, keep the reference zero mark on point O and randomly mark 3cm away from point O in all the direction.
 Mark as many points as u want away from point O, but all of them should be exactly 3 cm away from point O.
If you’ve selected sufficient points, you may notice that the shape is starting to resemble a circle and this is exactly what the definition of a circle is.
Circle Definition
A circle is a closed twodimensional figure in which the set of all the points in the plane is equidistant from a given point called “centre”. Every line that passes through the circle forms the line of reflection symmetry. Also, it has rotational symmetry around the centre for every angle. The circle formula in the plane is given as:
(xh)^{2} + (yk)^{2} = r^{2}
where (x,y) are the coordinate points
(h,k) is the coordinate of the centre of a circle
and r is the radius of a circle.
Circles Terminologies
Figure  Terms and Description 
AnnulusThe region bounded by two concentric circles. It is basically a ringshaped object.  
Arc – It is basically the connected curve of a circle
Sector – A region bounded by two radii and an arc. Segment A region bounded by a chord and an arc lying between the chord’s endpoints. It is to be noted that segments do not contain the centre 

Centre – It is the midpoint of a circle.
Chord A line segment whose endpoints lie on the circle Diameter A line segment having both the endpoints on the circle Radius A line segment connecting the centre of a circle to any point on the circle itself. Secant A straight line cutting the circle at two points. It is also called as an extended chord. Tangent A coplanar straight line touching the circle at a single point. 
Circle Formulas
Radius (r)  “A line segment connecting the centre of a
circle to any point on the circle itself “. The radius of the circle is denoted by “R” or “r” 

Diameter (d)  “ A line segment having both the endpoints
on the circle.” It is twice the length of radius i.e. d = 2r. From the diameter, the radius of the circle formula is obtained as r= d/2. 

Circumference (C)  The circumference of a circle is defined as the
distance around the circle. The word ‘perimeter’ is also sometimes used, although this usually refers to the distance around polygons, figures made up of the straight line segment. A circle circumference formula is given by C = πd = 2 π r Where, π = 3.1415 

Area (A)  Area of a circle is the amount of space occupied by the circle.
The circle formula to find the area is given by Area of a circle = πr^{2} 
Area of a circle proof
We know that Area is the space occupied by the circle.
Consider a concentric circle having external circle radius to be ‘r.’
Open all the concentric circle to form a rightangled triangle.
The outer circle would form a line having length 2πr forming the base.
The height would be ‘r’
Therefore the area of the rightangled triangle formed would be equal to the area of a circle.
Area of a circle = Area of triangle = (1/2) ×b ×h
= (1/2) × 2π r × r
Therefore, Area of a circle = πr^{2}
Circles Example Problem
Example Find the Area and the circumference of a circle whose radius is 10 cm. (Take the value of π = 3.14)
Solution Given: Radius = 10 cm. Area =π r^{2 }Square units = 3.14 × 10^{2} A= 314 cm^{2} Circumference, C = 2πr units C= 2 ×3.14× 10 Circumference= 62.8 cm 
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