A beautiful Parabolaaas

Geometry Level 3

A parabola y 2 = 4 a x y^2=4ax is drawn. Through two points , say 'P' and 'Q' on parabola which are also on latus rectum of that parabola , two lines are drawn which intersect at origin 'O'. so triangle POQ is formed. Now through points 'P' and 'Q' their normals are interscted at point 'R'. Now , through point 'R' , as a vertex another similar parabola is drawn in same direction. The points on latus rectum on this second parabola form triangle with origin . and similarly many parabolas are drawn. So, which parabola will have area of triangle formed in similar way, 1156 times the first one.

NOTE :-

  1. Always triangles are formed by the points on latus rectum of corresponding parabola and origin 'O'.

  2. New parabola is drawn whose vertex is at the point of intesction of normals drawn to points on latus rectum of previous one.

31 21 36 26

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1 solution

Akash Shukla
Dec 14, 2015

As, y 2 y^2 =4ax, co-ordinates of points on latus rectum are ‘P’ (a,2a) and ‘Q’(a,-2a) .

Equation of lines passing through (a,2a) and ‘O’ origin is y=2x.

Equation of lines passing through (a,-2a) and origin is y=-2x.

So they are ⊥ to each other. Hence area of equilateral triangle △POQ will be

1/2{ 1 2 a 2 + 4 a 2 1^2*a^2+4*a^2\ } =1/2{(5 a 2 a^2 )}

Now normal drawn to points ‘P’ and ‘Q’ have equations x+y=3a and x-y=3a respectively.

So they intersect at point (3a,0). Now a parabola is drawn whose vertex is at (3a,0) and focus is (4a,0) .

Also it is in the direction of previous one. So equation of this parabola is y 2 y^2 =4a(x-3a).

So points of latus rectum on this parabola are ‘A’(4a,2a) and ‘B’(4a,-2a). so area of equilateral triangle

△ AOB is 1/2{ 4 2 a 2 + 4 a 2 4^2*a^2+4*a^2\ }= 10 a 2 10a^2 .

Thus for ‘t’th parabola , area of triangle will be 1/2{ n 2 a 2 + 4 a 2 n^2*a^2+4*a^2\ }

Where , n=1+(t-1)(4-1).

Because x-co-ordinate will increase as 1,4,7,…..

So, n=1+3t-3=3t-2 ........................................... eq:-1

For area to be 1156 times

1/2{ n 2 a 2 + 4 a 2 n^2*a^2+4*a^2\ }=1156 1/2(5 a 2 a^2 )

This equals to , n 2 n^2 +4=5780, therefore , n=76

Therefore , putting in eqn 1

We get, t=26

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