# Conic Sections: Class 11 Mathematics NCERT Chapter 11

## Key Features of NCERT Material for Class 11 Maths Chapter 11– Conic Sections

In the last chapter 10, you learned about straight line. In this chapter you all will get to know about Conics.

### Quick revision notes

#### Circle

A circle is the set of all points in a plane, which are at a fixed distance from a fixed point in the plane. The fixed point is called the centre of the circle and the distance from centre to any point on the circle is called the radius of the circle.

The equation of a circle with radius r having centre (h, k) is given by (x – h)2 + (y – k)2 = r2.

The general equation of the circle is given by x2 + y2 + 2gx + 2fy + c = 0 , where, g, f and c are constants.

• The centre of the circle is (-g, -f).
• The radius of the circle is r =

The general equation of the circle passing through origin is x2 + y2 + 2gx + 2fy = 0.

The parametric equation of the circle x2 + y2 = r2 are given by x = r cos θ, y = r sin θ, where θ is the parametre and the parametric equation of the circle (x – h)2 + (y – k)2 = r2 are given by x = h + r cos θ, y = k + r sin θ.

Note: The general equation of the circle involves three constants which implies that at least three conditions are required to determine a circle uniquely.

#### Parabola

A parabola is the set of points P whose distances from a fixed point F in the plane are equal to their distance from a fixed line l in the plane. The fixed point F is called focus and the fixed line l is the directrix of the parabola.

 Forms of parabola y2= 4ax y2 = -4ax x2 = 4ay x2 = -4ay Axis of parabola y = 0 y = 0 x = 0 x = 0 Directrix of parabola x = -a x = a y = -a y = a Vertex (0, 0) (0, 0) (0, 0) (0, 0) Focus (a, 0) (-a, 0) (0, a) (0, -a) Length of latus rectum 4a 4a 4a 4a Focal length |x + a| |x – a| |y + a| |y – a|

#### Ellipse

An ellipse is the set of all points in a plane such that the sum of whose distances from two fixed points is constant.

or

An ellipse is the set of all points in the plane whose distances from a fixed point in the plane bears a constant ratio, less than to their distance from a fixed point in the plane. The fixed point is called focus, the fixed line a directrix and the constant ratio(e) the eccentricity of the ellipse. We have two standard forms of ellipse i.e.