I am a math teacher and my usual style of making curve sketching questions for my students is to beg, borrow, or steal from other sources. I decided recently to create my own problems in curve sketching of polynomials, only to run into the same problem over and over no matter what coefficients I chose (and the problem only existed if I chose coefficients that factored properly).
I am being a bit fussy in my choice of polynomials for f(x). The polynomials for f(x) and its first and second derivatives must have rational zeroes, no irrational or complex roots. f(x) itself is a quartic.
What follows is a methodology I thought would work, and since this is a small school I don't have easy access to colleagues who know more math than me, I have no idea as to why my idea appears to lead to lackluster results:
I know that if I have, say , then its subsequent derivatives exhibit the behaviour of factorials done partially, as in: ; and ; and finally: . This gave me the idea to work backwards from to to get a factorable second derivative. I would use synthetic division on an unknown constant term to get the completed polynomials for and as I worked backward, that I knew would factor using techniques familiar to my Grade 12 students.
The result is a polynomial f(x) with x-intercepts, max, min, and inflection points that can be found by either simple factoring or synthetic division, with therefore enough info to plot a reasonable graph. This sounds like an airtight idea, except that all I have gotten so far is an f(x) with a triple root and a single root, as in: Another one I found resulted in a triple root as well: .
Is this the best I can hope for with my conjecturing? Is there a way of getting a greater variety of roots with modifications to this method or by using an altogether different method? Remember: it must factor completely, no complex or irrational roots.