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Question

Let PQ be a variable focal chord of the parabola y2=4ax where vertex is A. Locus of , centroid of triangle APQ is a parabola P1 then vertex of parabola P1 is,

A
(2a3,0)
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B
(4a3,0)
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C
(8a3,0)
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D
(a3,0)
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Solution

The correct option is C (4a3,0)
Let P will be demanded are
P(t)=P(at2,2at)
& Q(t1,2at1)
For focal chord end points
t t1=1
t1=1t
Q(17)=Q⎜ ⎜ ⎜at2,2at⎟ ⎟ ⎟Let G(α,β) be centroid of APQ :
α=at2+ot23+o
β=2at+(2ot)+o3
3αa=t2+1t2 3β2a=(t1t)
3αa=(t2+1t22)+2
3αα=(t1t)2+2
3αa=(3β2a)2+2
12αa=9β2+8a2
Replacing α by x & β by y
9y2=12ax8a2
y2=4ax38a29
y2=4a3[x2a3]
The locus is a parabola with vertex at (=2a3,o) & locus reacts length is 4a3

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