Gujarati
Hindi
10-2. Parabola, Ellipse, Hyperbola
normal

Latus rectum of the conic satisfying the differential equation, $ x dy + y dx = 0$  and passing through the point $ (2, 8) $ is :

A

$4 \sqrt 2 $

B

$8$

C

$8 \sqrt 2 $

D

$16$

Solution

$\frac{{dy}}{y}\,\, + \,\frac{{dx}}{x}\, = \,0 $ $\Rightarrow$ $ ln$ $ xy = c$ $\Rightarrow$ $ xy = c$
passes through $(2,8)$ $ \Rightarrow$ $ c = 16$ 
  $xy =16$            $LR = 2a(e^2 – 1) = 2a$ 
solving with $y = x$ 
vertex is $(4, 4)$
distance from centre to vertex = $4\sqrt 2 \,$ 
$L.R. = $ length of $TA =8\sqrt 2 \,$ 

Standard 11
Mathematics

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