Question Details

If eccentricity of an ellipse

x 2 a 2 + y 2 b 2 = 1

which passes through which passes through point (3, 4) is √5/3 , then length of latus rectum of ellipse is

Options

A

45/3

B

85/3

C

47/3

D

87/3

Show Answer

Correct Answer :

Option B

85/3

Solution :

To find the length of the latus rectum of the given ellipse, we can follow these steps:

Step 1: Write down the standard equation of the ellipse and the eccentricity formula.
The equation of the ellipse is given by:
x2 a2 + y2 b2 = 1
The eccentricity e of an ellipse (assuming a>b) is related to its semi-major axis a and semi-minor axis b by the relation:
e2 = 1 - b2 a2

Step 2: Use the given eccentricity to find a relation between b2 and a2.
We are given that the eccentricity e=53. Substituting this value into the eccentricity equation:
( 5 3 ) 2 = 1 - b2 a2
59 = 1 - b2 a2
Rearranging the equation to solve for b2a2:
b2 a2 = 1 - 59 = 49
This gives us the relationship:
b2 = 49 a2

Step 3: Substitute the coordinates of the point (3, 4) into the ellipse equation.
Since the ellipse passes through the point (3,4), these coordinates must satisfy the equation of the ellipse:
32 a2 + 42 b2 = 1
9a2 + 16b2 = 1

Step 4: Solve for a2 and b2.
Substitute b2=49a2 into the equation from Step 3:
9a2 + 16 49 a2 = 1
Simplify the second term:
9a2 + 16×9 4a2 = 1
9a2 + 36a2 = 1
45a2 = 1
Thus, we find:
a2 = 45
Taking the positive square root for the semi-major axis length:
a = 45 = 3 5
Now, compute b2 using the relation from Step 2:
b2 = 49 × 45 = 20

Step 5: Calculate the length of the latus rectum.
The formula for the length of the latus rectum of an ellipse is:
Length of latus rectum = 2 b2 a
Substitute the values of b2 and a:
Length = 2 × 20 3 5 = 40 3 5
Rationalize the denominator by multiplying the numerator and denominator by 5:
Length = 40 5 3 × 5 = 8 5 3

Therefore, the length of the latus rectum of the ellipse is 853.

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