Q.

Let P(0, 6) and Q(0, – 4) be distinct points on parabola ‘C’ with focus O(0, 0). Let ‘L’ and ‘a’ denotes the length of latusrectum and length of intercept on x–axis by parabola C respectively. Then which of the following is/are TRUE ?

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a

The value of L is   485

b

The value of L is  245

c

The value of ‘a’ is 240

d

The value of ‘a’ is 120

answer is A, B.

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Detailed Solution

 y=mx+c is directrix of parabola S0,0,P0,6,Q0,4 SP=PM6=c61+m2______ (1) SQ=QM|4=c+41+m2______ (2) From (1) and (2) c=0  or24                        m=0  or±26 c=0  and  m=0(neglected) Directrix is y=26x24(or)y=26x24 Length of latusrectum = 4a=22a=2245=485 Equation of parabola is SP = PMx2+y2=26x+y+245 For xintercept of parabola put y = 0 x296x576=0 a=x1x2=240  

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