Engineering
Mathematics
Highlights on Parabola
Question

Let the curve C be the mirror image of the parabola y2 = 4x with respect to the line x + y + 4 = 0.  If A and B are the points of intersection of C with the line y = – 5, then the distance between A and B is

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Solution

Let image of (t2, 2t) in line x + y + 4 = 0 be (h, k).

 ht21=k2t1=2(t2+2t+4)12+12

⇒ h – t2 = k – 2t = – t2 – 2t – 4

⇒ h = – 2t – 4 and  k = – t2 – 4

 = (h+42)

 + 4 = (h+42)2

⇒ (h + 4)2 = – 4 (k + 4)

     Locus is (curve C)

  (x + 4)2 = – 4 (y + 4)

  For point of intersection with y = – 5

(x + 4)2 = – 4 (– 5 + 4) = 4

⇒ x + 4 = ± 2 ⇒ x = – 2 or – 6

  Distance AB will be 4.