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30 intersections (Posted on 2025-05-24) Difficulty: 3 of 5
Can the graphs of a polynomial of degree 20 and the function y=1/x40 have exactly 30 points of intersection?

No Solution Yet Submitted by Danish Ahmed Khan    
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Solution re: I would say - My solution | Comment 2 of 3 |
(In reply to I would say by Kenny M)

I will start with P(x)*(x^40)-1=0.  If this equation has 30 roots, then it must have at least 29 relative extremes, with one at least extreme occurring between each pair of consecutive roots.


The x-coordinates of those extremes occur at the roots of the derivative.
d/dx (P(x)*(x^40)-1) 
= 40*P(x)*(x^39) + P'(x)*(x^40)
= x^39 * [40*P(x) + P'(x)*x]

40*P(x) + P'(x)*x is a polynomial of degree 20, so it has at most 20 roots.  x^39 has only one root, x=0.  So then the derivative has at most 21 roots.  Not enough for the needed 29 roots that the derivative requires.  So the answer to the original question is "No".

  Posted by Brian Smith on 2025-05-24 22:55:13
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