RD Sharma class 9 maths chapter 9 comprises the various concepts of circles in a detailed manner. The circle is a round-shaped figure whose all points lie on the same plane and at an equal distance from a common point. This common point is called the centre of the circle. The line joining the centre to the circumference is called the Radius of the Circle. If the line starts from one point of the circumference, passes through the centre of the circle and ends at another point of the circumference, it is called the diameter. Basically Diameter = 2 x Radius. A Chord is a line between 2 points on the circular arc (circumference) without touching the centre of the circle. A circular wedge or a pie (in simple language) from the circle is called Sector. A line perpendicular to the radius that touches ONLY one point on the circumference is called "Tangent of the circle."
The circle area can be calculated by the formula
Area of a circle = πr2
where 'r' is the radius of the circle. And the value of 'π' is said to be 3.14159 (we'll take 3.14 for the sake of simplicity).
Also, the formula can be rewritten in terms of the diameter,
where 'd'' is the circle's diameter.
We can determine the circumference of the circles by the formula Circumference of a circle = 2πr or πd where ''r'' and ''d' are the radius and the diameter of the circle, respectively.
The chapter revolves around the various concepts of a circle, and the students get to learn about the concentric circle, the angle of an arc, the congruence of a circle and the arcs or sector. Ther students may need to determine the length of some random chord in the circle. In some of the problems here, the students have to know about Pythagoras's Theorem too.