Q.

If (1,–2) is the focus and +y-2=0 is the directix of the  ellipse 17x2-2xy+17y2-32x+76y+86=0 then its  eccentricity is

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a

1/5

b

1/2

c

1/3

d

1/4

answer is C.

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Detailed Solution

Given focus =(1, -2) and equation of directrix is x+y-2=0 

Now let P(x, y) be any point on ellipse then equation of ellipse is 

(x-1)2+(y+2)2=e2(x+y-2)22     

 2x2+1-2x+y2+4+4y=e2x2+y2+4+2xy-4y-4x 

 x2(2-e2)-2e2xy+(4e2-4)x+(2-e2)y2+(8+4e2)y+10-4e2=0   -(1)

Given equation of ellipse is 

17x2-2xy+17y2-32x+76y+86=0   -(2)

 since (1)&(2) represents same line then

2-e217=2e22=4e2-4-32  2-e2=17e2  18e2=2  e2=19  e=13

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