Find the equation of the ellipse in the following cases: (i) eccentricity e = ½ and foci (± 2, 0) (ii) eccentricity e = 2/3 and length of latus – rectum = 5
Find the equation of the ellipse in the following cases: (i) eccentricity e = ½ and foci (± 2, 0) (ii) eccentricity e = 2/3 and length of latus – rectum = 5

(i) 

    \[Eccentricity\text{ }e\text{ }=\text{ }{\scriptscriptstyle 1\!/\!{ }_2}~\]

and

    \[foci\text{ }\left( \pm \text{ }2,\text{ }0 \right)\]

Given:

Eccentricity

    \[e\text{ }=\text{ }{\scriptscriptstyle 1\!/\!{ }_2}\]

    \[Foci\text{ }\left( \pm \text{ }2,\text{ }0 \right)\]

Now let us find the equation to the ellipse.

We know that the equation of the ellipse whose axes are x and y – axis is given as
RD Sharma Solutions for Class 11 Maths Chapter 26 – Ellipse - image 47

By using the formula,

Eccentricity:

RD Sharma Solutions for Class 11 Maths Chapter 26 – Ellipse - image 48

    \[{{b}^{2}}~=\text{ }3{{a}^{2}}/4\]

It is given that foci

    \[\left( \pm \text{ }2,\text{ }0 \right)\text{ }=>foci\text{ }=\text{ }\left( \pm ae,\text{ }0 \right)\]

Where,

    \[ae\text{ }=\text{ }2\]

    \[a\left( 1/2 \right)\text{ }=\text{ }2\]

Or,

    \[a\text{ }=\text{ }4\]

    \[{{a}^{2}}~=\text{ }16\]

We know

    \[{{b}^{2}}~=\text{ }3{{a}^{2}}/4\]

    \[{{b}^{2}}~=\text{ }3\left( 16 \right)/4\]

So,

    \[=\text{ }12\]

So the equation of the ellipse can be given as

RD Sharma Solutions for Class 11 Maths Chapter 26 – Ellipse - image 49

    \[3{{x}^{2}}~+\text{ }4{{y}^{2}}~=\text{ }48\]

∴ The equation of the ellipse is

    \[3{{x}^{2}}~+\text{ }4{{y}^{2}}~=\text{ }48\]

(ii) eccentricity

    \[e\text{ }=\text{ }2/3~\]

and length of latus rectum

    \[=\text{ }5\]

Given:

Eccentricity

    \[e\text{ }=\text{ }2/3\]

Length of latus – rectum

    \[=\text{ }5\]

Now let us find the equation to the ellipse.

We know that the equation of the ellipse whose axes are x and y – axis is given as
RD Sharma Solutions for Class 11 Maths Chapter 26 – Ellipse - image 50

By using the formula,

Eccentricity:

RD Sharma Solutions for Class 11 Maths Chapter 26 – Ellipse - image 51

By using the formula, length of the latus rectum is

    \[2{{b}^{2}}/a\]

RD Sharma Solutions for Class 11 Maths Chapter 26 – Ellipse - image 52

So the equation of the ellipse can be given as

RD Sharma Solutions for Class 11 Maths Chapter 26 – Ellipse - image 53

    \[20{{x}^{2}}~+\text{ }36{{y}^{2}}~=\text{ }405\]

∴ The equation of the ellipse is

    \[20{{x}^{2}}~+\text{ }36{{y}^{2}}~=\text{ }405\]