1)

Let a,r, s, t be non-zero real numbers. Let  P( at2,2at) , Q , R (ar2, 2ar) and S( as2, 2as) be distinct point on the parabola y2=4ax . Suppose  that PQ is the  focal  chord and  lines QR and PK are parallel, where K is the point (2a,0)

The value of r is 


A) 1t

B) t2+1t

C) 1t

D) t21t

Answer:

Option D

Explanation:

 Plan 

 (i)  P( at2, 2at) is one end point of focal chord of parabola y2=4ax,

 then other end point is   (at2,2at)

 (ii)  Slope of line joining two points  (x1,y1) and (x2,y2) is given by  y2y1x2x1

If PQ is foacl  chord, then coordinates of Q will be (at2,2at)

 Now, slope of QR= slope of PK

  2ar+2a/tar2a/t2=2atat22a

     r+1/tr21/t2=tt22

     1r1/t=tt22

     r1t=t22t=t2t

       r=t1t=t21t