         6.12 Circles - Part 3:

6.12.1: Arcs of a circle

Two arcs of two different  circles having same radii are said to be ‘congruent’ if their central angles are same.

Arc ASB = Arc CTD if AOB = CO’D

6.12.1 Theorem 1: If two arcs are congruent then their chords are equal

To prove: AB=CD

Proof:

 1. OA = O’C, OB = O’D (Radii) 2. AOB = CO’D (it is given that arcs are congruent) Hence by SAS Postulate on congruence AOB  CO’D Hence AB = CD 6.12.1 Theorem 2: If two chords of circles having same radii are same,

then their arcs are congruent.

Note: This is converse of the previous theorem.

Use SSS postulate to show that AOB = CO’D

6.12.1: Areas of sectors/segments of circle

 If ‘r’ is the radius of a circle, we know that the circumference and area of the circle are given by Circumference of the circle = 2 r, Area of the circle = r2,                                  Where is a constant whose approximate value we use for our calculations is 22/7 (3.1428). If (where is in degrees) is the angle at center ( COD) formed by the arc CSD then 1. Length of the arc CSD = ( /180) * r 2. Area of the sector CSDO (shaded portion in the adjoining figure) = ( /360) * r2 = ( /180) *( r*r)/2 = {( /180) * r}*(r/2) = Length of the arc*(radius/2) Note: radians = 1800 and x0 = (x* )/180 radians Let AOB = in the adjoining figure with AB as chord We note that Area of triangle ABO = (1/2)*base*height = (1/2)*BO*AM = (1/2) *r*rsin = (1/2) r2*sin (AM = rsin : Refer to section 7.1 for definition of sin of an angle)   From the figure we notice that Area of Sector ASBO = Area of triangle ABO + Area of segment ASB Area of segment ASB = Area of Sector ASBO - Area of triangle ABO = ( /360) * r2 - (1/2) r2*sin = r2 {( * /360) - (sin /2)} Note: For all the above calculations must be in degrees. 6.12 Summary of learning

 No Points to remember 1 Congruency of arcs 2 Formula for length of an arc, area of an arc, Area of a segment         