Talk:Cubic function/Archive 5

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Archive 1

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Archive 3

Error in "Q" in the monic trinomial form

There is an error in the formula for "q" in the monic trinomial form.

The denominator should be 27a^2 instead of 27a^3. please someone correct this. You can see a link here:

http://www.sosmath.com/algebra/factor/fac11/fac11.html

Or you can derive your own expression (which I did too) — Preceding unsigned comment added by 140.105.70.136 (talk) 15:08, 25 May 2012 (UTC)

The formula is correct. In your link the polynomial in y is not monic, as y³ has a as coefficient. Thus you have to divide by a the equation of the link before comparing. D.Lazard (talk) 17:01, 25 May 2012 (UTC)

Geometric interpretation

You don't need perfect vision to see that the angles θ are not the same, or that the three red lines intersection points on the circle do not form an equilateral triangle. Ssscienccce (talk) 10:41, 6 September 2012 (UTC)

Good catch. I modified the figure by moving the top red line slightly clockwise to make the angles match, and to make it look more like an equilateral triangle. - DVdm (talk) 11:33, 6 September 2012 (UTC)

In the illustration of Omar Khayyám's geometric solution, the circle is drawn so that the parabola intersects it at the top, putting the vertical line through the center of the circle. This suggests a trivial solution, x=b/(2a^2). It would be better to draw the circle with a different diameter (choose a different value of b) so as not to give this false impression. — Preceding unsigned comment added by 64.95.214.18 (talk) 18:55, 19 December 2012 (UTC)