If the vertex of the parabola is at the point (–3, 0) and the directrix is the line x+5=0 , then its equation is

(a) y 2 = − 8 ( x + 3 )
(b) x 2 = 8 ( y + 3 )
(c) y 2 = 8 ( x + 3 )
(d) y 2 = 8 ( x + 5 )

5 Views|Posted a year ago
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1 Answer
A
a year ago

This is an objective Type Questions as classified in NCERT Exemplar

Sol:

 

Given  that vertex=(−3,0)∴  a=−3  and  directrix  is  x+5=0According  to  the  definition  of  the  parabola,  we  getAF=AD  i.e.,  A  is  the mid  point of  DF∴                    −3=x1−52         ⇒x1=−6+5=−1and                  0=0+y12         ⇒y1=0∴       Focus  F=(−1,0)Now  (x+1)2+(y−0)2=|x+512+02|Squaring  both  sides,  we  get               (x+1)2+(y−0)2=(x+5)2⇒               x2+1+2x+y2=x2+25+10x⇒                                         y2=10x−2x+24         ⇒y2=8x+24⇒                                         y2=8(x+3)Hence,  the  correct  option  is  (a).

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