 # Ellipse

An ellipse is the locus of all those points in a plane such that the sum of their distances from two fixed points in the plane is constant. The fixed points are known as the foci (singular focus), which are surrounded y the curve. The shape of the ellipse is in an oval shape and the area of ellipse is defined by its major axis and minor axis. Eccentricity is a factor of the ellipse, which demonstrates the elongation of it and is denoted by ‘e’.

Ellipse is similar to other parts of the conic section such as parabola and hyperbola, which are open is the shape and unbounded. The ellipse is defined by its equation, which we will learn here in this article, along with its formula of area of the ellipse.

## Definition of Ellipse

An ellipse if we speak in terms of locus, it is the set of all points on a XY-plane, whose distance from two fixed points (known as foci) adds up to a constant value.

A circle is also an ellipse, where the foci are at the same point, which is the center of the circle.

Ellipse is defined by its two-axis along x and y-axis: Major axis and Minor Axis. The major axis is the longest diameter of the ellipse, going through the center from one end to the other, at the broad part of ellipse. Whereas the minor axis is the shortest diameter of ellipse, crossing through the center at the narrowest part.

## Eccentricity of the Ellipse

The ratio of distances from the center of the ellipse from either focus to either of the vertices of the ellipse is defined as the eccentricity of the ellipse.

The eccentricity of ellipse, e = c/a

Since c ≤ a the eccentricity is always greater than 1 in the case of an ellipse.

## Ellipse Equation

When the centre of the ellipse is at the origin (0,0) and the foci are on x-axis and y-axis, then we can easily derive the ellipse equation.

The equation of the ellipse is given by;

x2/a2 + y2/b2 = 1

### Derivation of Ellipse Equation

Now, let us see, how it is derived. .

The above figure represents an ellipse such that P1F1 + P1F2 = P2F1 + P2F2 = P3F1 + P3F2 is a constant. This constant is always greater than the distance between the two foci. When both the foci are joined with the help of a line segment then the mid-point of this line segment joining the foci is known as the center, O represents the center of the ellipse in the figure given below: The line segment passing through the foci of the ellipse is the major axis and the line segment perpendicular to the major axis and passing through the center of the ellipse is the minor axis. The end points A and B as shown are known as the vertices which represent the intersection of major axis with the ellipse. ‘2a’ denotes the length of the major axis and ‘a’ is the length of the semi-major axis. ‘2b’ is the length of the minor axis and ‘b’ is the length of the semi-minor axis. ‘2c’ represents the distance between two foci.

Let us consider the end points A and B on the major axis and points C and D at the end of the minor axis. The sum of distances of B from F1 is F1B + F2B = F1O + OB + F2B (From the above figure)

⇒ c + a + a – c = 2a

The sum of distances from point C to F1 is F1C + F2C

⇒ F1C + F2C = √(b2 + c2) + √(b2 + c2) = 2√(b2 + c2)

By definition of ellipse;

2√(b2 + c2) = 2a

a = √(b2 + c2)

⇒ a2 = b2 + c2

c2 = a2 – b2

Special Cases:

• If c = 0 then F1 and F2e. both foci merge together with center of ellipse. Also a2 becomes equal to b2, i.e. a = b so now we get a circle in this case.
• If c = a then b becomes 0 and we get a line segment F1F2.

### Standard Equation of Ellipse

The simplest method to determine the equation of an ellipse is to assume that centre of the ellipse is at the origin (0, 0) and the foci lie either on x- axis or y-axis of the Cartesian plane as shown below: Both the foci lie on x- axis and center O lies at the origin.

Let us consider the figure (a) to derive the equation of an ellipse. Let the coordinates of F1 and F2 be (-c, 0) and (c, 0) respectively as shown. Let us consider a point P(x, y) lying on the ellipse such that P satisfies the definition i.e. the sum of distances of P from F1 and F2 in the plane is a constant 2a.

⇒ PF1 + PF2 = 2a – – – (1)

Using distance formula the distance can be written as: Squaring and simplifying both sides we get;  Now since P lies on the ellipse it should satisfy equation 2 such that 0 < c < a. Thus, On simplifying,

PF1 = a + (c/a)x

Similarly,

PF= a – (c/a)x

Therefore,

PF+ PF= 2a

Therefore the equation of the ellipse with centre at origin and major axis along the x-axis is: where –a ≤ x ≤ a.

Similarly, the equation of the ellipse with center at origin and major axis along the y-axis is: where –b  ≤ y ≤ b.

## Ellipse Formula

As we know, an ellipse is a closed-shape structure in a two-dimensional plane. Hence, it covers a region in a 2D plane. So, this bounded region of the ellipse is its area.  The shape of the ellipse is different from circle, hence the formula for its area will also be different.

### Area of Ellipse

Area of the circle is calculated based on its radius, but the area of the ellipse depends on the length of the minor axis and major axis.

Area of the circle = πr2

And,

Area of the ellipse = π x Major Axis x Minor Axis

 Area of the ellipse = π.a.b

where a and b are the length of the minor axis and major axis.

### Latus Rectum

The line segments perpendicular to the major axis through any of the foci such that their endpoints lie on the ellipse are defined as the latus rectum. The length of latus rectum is 2b2/a.

L = 2b2/a

where a and b are the length of the minor axis and major axis.

### Example of Ellipse

Q.1: If the length of the major axis is 7cm and the minor axis is 5cm of an ellipse. Find its area.

Solution: Given, length of the major axis of an ellipse = 7cm

length of the minor axis of an ellipse = 5cm

By the formula of area of an ellipse, we know;

Area = π x major axis x minor axis

Area = π x 7 x 5

Area = 35 π

or

Area = 35 x 22/7

Area = 110 cm2