# Ellipse Formulae

Ellipse formulae will help us to solve different types of problems on ellipse in co-ordinate geometry.

1. $$\frac{x^{2}}{a^{2}}$$ + $$\frac{y^{2}}{b^{2}}$$ = 1, (a > b)

(i) The co-ordinates of the centre are (0, 0).

(ii) The co-ordinates of the vertices are (± a, 0) i.e., (-a, 0) and (a, 0).

(iii) The co-ordinates of the foci are (± ae, 0) i.e., (- ae, 0) and (ae, 0)

(iv) The length of major axis = 2a and the length of minor axis = 2b.

(v) The major axis is along x axis and the equations of major axes is y = 0.

(vi) The minor axis is along y axis and the equations of minor axes is x = 0.

(vii) The equations of the directrices are: x = ± $$\frac{a}{e}$$ i.e., x = - $$\frac{a}{e}$$ and x = $$\frac{a}{e}$$.

(viii) The eccentricity of the ellipse is b$$^{2}$$ = a$$^{2}$$(1 - e$$^{2}$$) or, e = $$\sqrt{1 - \frac{b^{2}}{a^{2}}}$$.

(ix) The length of the latus rectum 2 $$\frac{b^{2}}{a}$$ = 2a(1 - e$$^{2}$$).

(x) The distance between the two foci = 2ae.

(xi) The distance between two directrices = 2 $$\frac{a}{e}$$.

(xii) Focal distances of a point (x, y) are a ± ex

(xiii) The co-ordinates of the four ends of latera recta are (ae, $$\frac{b^{2}}{a}$$), (ae, -$$\frac{b^{2}}{a}$$), (- ae, $$\frac{b^{2}}{a}$$) and (- ae, -$$\frac{b^{2}}{a}$$).

(xiv) The equations of latera recta are x = ± ae i.e., x = ae and x = -ae.

2. $$\frac{x^{2}}{b^{2}}$$ + $$\frac{y^{2}}{a^{2}}$$ = 1, (a > b)

(i) The co-ordinates of the centre are (0, 0).

(ii) The co-ordinates of the vertices are (0, ± a) i.e., (0, -a) and (0, a).

(iii) The co-ordinates of the foci are (0, ± ae) i.e., (0, - ae) and (0, ae)

(iv) The length of major axis = 2a and the length of minor axis = 2b.

(v) The major axis is along Y-axis and the equations of major axes is x = 0.

(vi) The minor axis is along X-axis and the equations of minor axes is y = 0.

(vii) The equations of the directrices are: y = ± $$\frac{a}{e}$$ i.e., y = - $$\frac{a}{e}$$ and y = $$\frac{a}{e}$$.

(viii) The eccentricity of the ellipse is b2 = a$$^{2}$$(1 - e$$^{2}$$) or,  e = $$\sqrt{1 - \frac{b^{2}}{a^{2}}}$$

(ix) The length of the latus rectum 2 $$\frac{b^{2}}{a}$$ = 2a (1 - e$$^{2}$$).

(x) The distance between the two foci = 2ae.

(xi) The distance between two directrices = 2 $$\frac{a}{e}$$.

(xii) Focal distances of a point (x, y) are a ± ey

(xiii) The co-ordinates of the four ends of latera recta are ($$\frac{b^{2}}{a}$$, ae), (-$$\frac{b^{2}}{a}$$, ae), ($$\frac{b^{2}}{a}$$, -ae) and (-$$\frac{b^{2}}{a}$$, -ae).

(xiv) The equations of latera recta are y = ± ae i.e., y = ae and y = -ae.



3. $$\frac{(x - α)^{2}}{a^{2}}$$ + $$\frac{(y - β)^{2}}{b^{2}}$$ = 1, (a > b)

(i) The co-ordinates of the centre are (α, β).

(ii) The co-ordinates of the vertices are (α ± a, β) i.e., (α - a, β) and (α + a, β).

(iii) The co-ordinates of the foci are (α ± ae, β) i.e., (α - ae, β) and (α + ae, β)

(iv) The length of major axis = 2a and the length of minor axis = 2b.

(v) The major axis is along parallel to x axis and the equations of major axes is y = β.

(vi) The minor axis is along parallel to y axis and the equations of minor axes is x = α.

(vii) The equations of the directrices are: x = α ± $$\frac{a}{e}$$ i.e., x = α - $$\frac{a}{e}$$ and x = α + $$\frac{a}{e}$$.

(viii) The eccentricity of the ellipse is b$$^{2}$$ = a$$^{2}$$(1 - e$$^{2}$$) or, e =$$\sqrt{1 - \frac{b^{2}}{a^{2}}}$$

(ix) The length of the latus rectum 2  $$\frac{b^{2}}{a}$$ = 2a (1 - e$$^{2}$$).

(x) The distance between the two foci = 2ae.

(xi) The distance between two directrices = 2  $$\frac{a}{e}$$.

4. $$\frac{(x - α)^{2}}{b^{2}}$$ + $$\frac{(y - β)^{2}}{a^{2}}$$ = 1, (a > b)

(i) The co-ordinates of the centre are (α, β).

(ii) The co-ordinates of the vertices are (α, β ± a) i.e., (α, β - a) and (α, β + a).

(iii) The co-ordinates of the foci are (α, β ± ae) i.e., (α, β - ae) and (α, β + ae).

(iv) The length of major axis = 2a and the length of minor axis = 2b.

(v) The major axis is along parallel to Y-axis and the equations of major axes is x = α.

(vi) The minor axis is along parallel to X-axis and the equations of minor axes is y = β.

(vii) The equations of the directrices are: y = β ± $$\frac{a}{e}$$ i.e., y = β - $$\frac{a}{e}$$ and y = β + $$\frac{a}{e}$$.

(viii) The eccentricity of the ellipse is b$$^{2}$$ = a$$^{2}$$(1 - e$$^{2}$$) or, e = $$\sqrt{1 - \frac{b^{2}}{a^{2}}}$$

(ix) The length of the latus rectum 2  $$\frac{b^{2}}{a}$$ = 2a (1 - e$$^{2}$$).

(x) The distance between the two foci = 2ae.

(xi) The distance between two directrices = 2  $$\frac{a}{e}$$.

5. The point P (x$$_{1}$$, y$$_{1}$$) lies outside, on or inside the ellipse $$\frac{x^{2}}{a^{2}}$$ + $$\frac{y^{2}}{b^{2}}$$ = 1 according as $$\frac{x_{1}^{2}}{a^{2}}$$ + $$\frac{y_{1}^{2}}{b^{2}}$$ – 1 > 0, = or < 0.

6. If $$\frac{x^{2}}{a^{2}}$$ + $$\frac{y^{2}}{b^{2}}$$ = 1 is an ellipse, then its auxiliary circle is x$$^{2}$$ + y$$^{2}$$ = a$$^{2}$$.

7. The equations x = a cos ф, y = b sin ф taken together are called the parametric equations of the ellipse $$\frac{x^{2}}{a^{2}}$$ + $$\frac{y^{2}}{b^{2}}$$ = 1

8. The co-ordinates of the point having eccentric angle ф can be written as (a cos ф, b sin ф). Here (a cos ф, b sin ф) are known as the parametric co-ordinates of the point P.