Q.

 P is point on y2=12x such that its focal distance is 6. Tangent drawn at P intersect tangent at vertex at point T (y > 0). Point R is on x-axis inside parabola such that SR = 6 (S is focus). M is a point on parabola such that tangents at M & P meet at point N such that SN = 5, then which of the following is/are TRUE?

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a

Sum of squares of possible ordinates of M is 28

b

Area of quadrilateral PRST is 18 units

c

Sum of square of possible ordinates of M is 30

d

Area of quadrilateral PRST is 27 units

answer is B, C.

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Detailed Solution

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As  SP=6=3t2+3t=1 P=(3,6)       Tangent at P: y = 6 (x + 3)      xy+3=0 T=(0,3)ST=32        Also  Q=(3,0)QS=6

As given SR = 6  R(9,0)

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As we know if tangent and normal at any point P cuts axis at Q & R then SP = SQ = SR
According to figure PR is normal
Now,
Area of PRST = Area of ΔPQR - Area of  ΔTQS
=12×PS×QR12×OT×QS=12×6×1212×3×6=27
Also as we know if tangent at P & M meet at point N, then
(SN)2=SP×SM       25=6×SM

SM=256=3t2+3t2=718t=±732

Ordinate of M = 2at

=2.3.(±732)=±14

sum of squares = 14 + 14 = 28

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