Choose any point of this hyperbola

Geometry Level 3

What is the area of the triangle delimited by any line tangent to the hyperbola x y = 2018 xy = 2018 and the coordinate axes?

Note.- My apologies for the picture.

Assumption.- the tangent line to the hyperbola passes through any point of the hyperbola


The answer is 4036.

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1 solution

Tom Engelsman
Jun 23, 2018

Let y = 2018 x y = \frac{2018}{x} be our hyperbola in question whose derivative equals y = 2018 x 2 y' = -\frac{2018}{x^2} . Let P ( x 0 , 2018 x 0 ) P(x_{0}, \frac{2018}{x_{0}}) , where x 0 > 0 , x_{0} > 0, be any point on the hyperbola where the tangent line is expressible as:

y 2018 x 0 = 2018 x 0 2 ( x x 0 ) y = 2018 x 0 2 x + 4036 x 0 y - \frac{2018}{x_{0}} = -\frac{2018}{x_{0}^{2}} (x - x_{0}) \Rightarrow y = -\frac{2018}{x_{0}^{2}} x + \frac{4036}{x_{0}} (i).

Solving for the x and y-intercepts of this tangent line gives ( x , y ) = ( 2 x 0 , 0 ) ; ( 0 , 4036 x 0 ) (x,y) = (2x_{0}, 0 ); (0, \frac{4036}{x_{0}}) , and the area of the right triangle contained between the coordinate axes + the tangent line equals:

A = 1 2 ( 2 x 0 ) ( 4036 x 0 ) = 4036 . A = \frac{1}{2} \cdot (2x_{0})(\frac{4036}{x_{0}}) = \boxed{4036}.

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