Which is the nearest one?

Geometry Level 3

Which point ( x , y ) (x,y) on the straight line x + y = 4 x+y = 4 is nearest to the parabola y 2 = 4 ( x 10 ) y^2 = 4(x-10) ?

( 3 2 , 5 2 ) \left(\frac32,-\frac52 \right) ( 17 2 , 9 2 ) \left(\frac{17}2,\frac92 \right) ( 17 2 , 9 2 ) \left(\frac{17}2,-\frac92 \right) ( 3 2 , 5 2 ) \left(\frac32,\frac52 \right)

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1 solution

Rohit Ner
Jun 1, 2016

The point of intersection of the given line and the common normal will be the required point.

Common normal will have slope m = 1 m=1 whose equation is obtained as follows,

y 2 = 4 ( x 10 ) 2 y y = 4 1 y = y 2 m = y 2 ( x , y ) = ( 11 , 2 ) y + 2 x 11 = 1 y = x 13 \begin{aligned} {y}^2=4(x-10)\\2yy'&=4\\-\dfrac{1}{y'}&=-\dfrac{y}{2}\\m&=-\dfrac{y}{2}\\\Rightarrow (x,y)&=(11,-2)\\\dfrac{y+2}{x-11}&=1\\y&=x-13\end{aligned}

The required point is obtained by solving the line x + y = 4 x+y=4 and x y = 13 x-y=13 which is ( 17 2 , 9 2 ) \large \color{#3D99F6}{\boxed{\left(\dfrac{17}{2},-\dfrac{9}{2}\right)}} .

Nice solution,+1!

Rishabh Tiwari - 5 years ago

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