Least Distance

Geometry Level 3

On the parabola y = x 2 y={ x }^{ 2 } , the point least distant from the straight line y = 2 x 1 y=2x-1 is

(1,0) (1,-1) (0,0) (1,1)

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2 solutions

Vishnu Bhagyanath
May 17, 2015

If any point coincides, then it is the closest.

Checking if there is a real number solution for the two equations x 2 2 x + 1 = 0 x^2 - 2x +1 = 0 ( x 1 ) 2 = 0 (x-1)^2=0 When x is 1, y is 1. Point (1,1) is satisfying both equations.

Ayush Bhaskar
Aug 16, 2015

Let the point on the parabola y = x 2 {y}={x}^{2} be ( x , x 2 ) ({x},{x}^{2} ) .

Let the point on the straight line y = 2 x 1 y=2x-1 be ( x , 2 x 1 ) ({x},2{x}-1) .

Using the distance formula, we get the distance between the two points as:

( x x ) 2 + ( x 2 2 x + 1 ) 2 \sqrt{(x-x)^{2}+(x^{2}-2x+1)^{2}}

= ( x 2 2 x + 1 ) 2 =\sqrt{(x^{2}-2x+1)^{2}}

= x 2 2 x + 1 =x^{2}-2x+1

To get the minimum distance, we use the concept of minima-maxima.

that is, d d x ( x 2 2 x + 1 ) = 0 \dfrac{\text{d}}{\text{d}x} (x^{2}-2x+1) = 0

2 x 2 = 0 \implies 2x-2=0

x = 1 \implies x=1

Thus the point on the parabola is ( 1 , 1 ) \boxed {(1,1) } .

Moderator note:

Your solution assumes that the "closest points" must lie on a vertical line. This need not be true for any 2 random curves. It just so happens that these curves touch, which is why your solution appears to work.

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