NCERT Class XI Mathematics - Conic Sections - Solutions

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Question : 31
Total: 71
x2
25
+
y2
100
= 1
Solution:  
Given equation of ellipse is
x2
25
+
y2
100
= 1
Clearly, 100 > 25
The equation of ellipse in standard form is
y2
a2
+
x2
b2
= 1
∴ a2 = 100 ⇒ a = 10 and b2 = 25 ⇒ b = 5
We know that c = √a2−b2 ⇒ c = √100−25 = √75 = 5√3
∴ Coordinates of foci are (0, ±c) i.e. (0, ± 5 √3)
Coordinates of vertices are (0, ±a) i.e. (0, ±10).
Length of major axis = 2a = 2 × 10 = 20
Length of minor axis = 2b = 2 × 5 = 10
Eccentricity (e) =
c
a
=
5√3
10
=
√3
2

Length of latus rectum =
2b2
a
=
2×25
10
= 5.
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