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Geometry Level 4

Let y 2 = x y^2 = x be a given parabola. Let a variable chord cut parabola at points P P and Q Q . Also let C C be the vertex of the parabola.

Given that locus of intersection of tangents at P P and Q Q is x + 1 = 0 x+1=0 . Let minimum area of of triangle P C Q PCQ be M M .

Find the value of 4 M 4M .


The answer is 4.

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1 solution

Let the points P & Q be ( a t 1 2 , 2 a t 1 ) (at^{2}_{1},2at_{1}) & ( a t 2 2 , 2 a t 2 ) (at^{2}_{2},2at_{2}) .

Here a a = = 1 4 \frac{1}{4} ,

Therefore the points are P ( t 1 2 4 , t 1 2 ) (\frac{t^{2}_{1}}{4},\frac{t_{1}}{2}) & Q ( t 2 2 4 , t 2 2 ) (\frac{t^{2}_{2}}{4},\frac{t_{2}}{2})

So the point of contact of the tangents is ( a t 1 t 2 , a ( t 1 + t 2 ) ) (at_{1}t_{2},a(t_{1} + t_{2}))

So, point of contact is ( t 1 t 2 4 , t 1 + t 2 4 ) (\frac{t_{1}t_{2}}{4},\frac{t_{1}+t_{2}}{4})

As the point of contact lies on the line x x + + 1 1 = = 0 0

So t 1 t 2 t_{1}t_{2} = = 4 -4

If t 1 t_{1} = = t t then t 2 t_{2} = = 4 t \frac{-4}{t}

So the points are P ( t 2 4 , t 2 ) (\frac{t^{2}}{4},\frac{t}{2}) & Q ( 4 t 2 , 2 t ) (\frac{4}{t^{2}},\frac{-2}{t})

So the area of the triangle is given by,

1 2 \frac{1}{2} ( x 1 y 1 x 2 y 2 \begin{vmatrix} x_{1} & y_{1} \\ x_{2} & y_{2}\end{vmatrix} + + x 2 y 2 x 3 y 3 \begin{vmatrix} x_{2} & y_{2} \\ x_{3} & y_{3}\end{vmatrix} + + x 3 y 3 x 1 y 1 \begin{vmatrix} x_{3} & y_{3} \\ x_{1} & y_{1}\end{vmatrix} )

So by substituting the values we get,

Area = = 1 t \frac{1}{t} + + t 4 \frac{t}{4}

So area is maximum when t = 2 t= 2 .

So the area is equal to 1 1 .

So 4 M 4M = = 4 4

Cant you just say: x=1, therefore, the Y is +-1 so you have a triangle set up at (0,0) (1,1) (1,-1) and the area is 1 (1 1 0.5*2) so 4M = 4

Jordan Shemilt - 3 years, 2 months ago

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