Q48 / 90 · JEE Main 2022 (25 Jun Shift 1) · Mathematics

Let x=2t,y=t23 be a conic. Let S be the focus and B be the point on the axis of the conic such that SA⊥BA, where A is any point on the conic. If k is the ordinate of the centroid of the ΔSAB, then limt→1k is equal to

  • A
    1718
  • B
    1918
  • C
    1118
  • Correct answer
    D
    1318

Solution

The parametric form x=2t,y=t23 represents the parabola x2=12y

Figure

 

Given AS⊥AB

So, mAS·mAB=-1

⇒3-t230-2t·α-t230-2t=-1

 ⇒3α=27t2+t4t2-9

Ordinate of centroid of ΔSAB=k=α+t23+33=9+3α+t29

limt→1k=limt→1199+t2+27t2+t4t2-9=1318

Hence, the correct answer is option: 'D'.

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