Intersection points on circle

Geometry Level 5

The parabola f ( x ) = x 2 f(x)=x^2 intersects the graph of g ( x ) = x 4 + a x 3 2 x 2 + b x + 1 g(x) = x^4 + ax^3 -2x^2+ bx +1 at four distinct points. These four points on a same circle. Find the value of 1000 a b \lfloor{1000ab}\rfloor .


This problem is part of Curves... cut or touch?


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1 solution

Chan Lye Lee
Nov 13, 2015

Suppose the equation of the circle is ( x h ) 2 + ( y k ) 2 = r 2 (x-h)^2+(y-k)^2=r^2

Since it intersects with the graph f ( x ) = x 2 f(x)=x^2 , the above equation can be rewritten as ( x h ) 2 + ( x 2 k ) 2 = r 2 (x-h)^2+(x^2-k)^2=r^2

Note that the coefficient of x 3 x^3 of above quartic equation is 0.

Let the x x -coordinates of 4 intersection points be α 1 , α 2 , α 3 \displaystyle \alpha_1, \alpha_2, \alpha_3 and α 4 \displaystyle\alpha_4 . Then α 1 + α 2 + α 3 + α 4 = 0 \alpha_1+\alpha_2+\alpha_3+\alpha_4 = \color{#D61F06}0 . On the other hand, from g ( x ) = x 4 + a x 3 2 x 2 + b x + 1 g(x) = x^4 + ax^3 -2x^2+ bx +1 , the sum of roots is a 1 \displaystyle -\frac{a}{1} . So a = 0 \displaystyle a=0 and hence 1000 a b = 0 \lfloor{1000ab}\rfloor=0 .

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