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Imaginary triangles (Posted on 2021-09-24) Difficulty: 4 of 5
Consider the set of cubic polynomials, x3 + (1-p)x2 + px + 1, p ∈ Z. Then consider the set of triangles formed by the roots of these polynomials in the complex plane. Find the value of p which maximizes the area of such triangles.

No Solution Yet Submitted by Danish Ahmed Khan    
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Some Thoughts A start. Stuck. | Comment 1 of 5
Call the cubic polynomial z3 + (1-p)z2 + pz + 1, p ∈ Z

then its solutions are of the form z=(x+yi) with real x,y

If we make this substitution we get 
(x+yi)3 + (1-p)(x+yi)2 + p(x+yi) + 1 = 0
a polynomial that can be separated into real and imaginary parts:
Re: x^3-3xy^2+(1-p)(x^2-y^2)+px+1=0
Im: 3x^2y-y^3+(1-p)2xy+py=0

These equations can be plotted, complex plane becomes the (x,y) plane:  https://www.desmos.com/calculator/fggwdkex9w

The complex roots of the original polynomial are the intersections of the real (red) and imaginary (green) plus the x-intercept of the real (red).

Now we can at least see the triangle's vertices.  From here I'm stuck.  call the vertices (a,b), (a,-b), (c,0).  I need to find a,b,c in terms of p and the area becomes (a-c)b. 




  Posted by Jer on 2021-09-28 08:38:55
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