Does y = ax^3 + bx^2 + cx + d Have Two Distinct Turning Points if b^2 > 3ac?

  • Context: Graduate 
  • Thread starter Thread starter disfused_3289
  • Start date Start date
  • Tags Tags
    Geometrical Proof
Click For Summary

Discussion Overview

The discussion revolves around the conditions under which the cubic polynomial y = ax^3 + bx^2 + cx + d has two distinct turning points, specifically examining the inequality b^2 > 3ac. Participants also explore finding specific values of a, b, c, and d that yield turning points at given coordinates.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant asserts that the condition for two distinct turning points is met when b^2 > 3ac, deriving this from the discriminant of the derivative of the cubic function.
  • Another participant provides a method to find specific values of a, b, c, and d that result in turning points at (0.5, 1) and (1.5, -1), leading to a system of equations.
  • There is a mention of the relationship between the roots of the derivative and the coefficients a, b, and c, indicating that the sum and product of the roots can be expressed in terms of these coefficients.
  • A later reply indicates a misunderstanding of a previous statement, suggesting that not all participants are clear on the mathematical concepts discussed.
  • One participant expresses gratitude for clarification regarding the discriminant, indicating a learning moment within the discussion.

Areas of Agreement / Disagreement

Participants generally agree on the mathematical condition b^2 > 3ac for the existence of two distinct turning points, but there is no consensus on the correctness of the specific values of a, b, c, and d proposed by one participant.

Contextual Notes

The discussion includes assumptions about the nature of the cubic function and its derivatives, as well as dependencies on the specific values chosen for a, b, c, and d. The resolution of the system of equations remains unresolved.

Who May Find This Useful

Students and enthusiasts of mathematics, particularly those interested in polynomial functions and their properties, may find this discussion beneficial.

disfused_3289
Messages
12
Reaction score
0
Prove that the graph of y= ax^3 + bx^2 + cx + d has two distinct turning points if
b^2> 3ac. Find values of a,b,c and d for which the graph of this form has turning points at (0.5, 1) and (1.5, -1)
 
Physics news on Phys.org
hi iam a new member to this site

well here isolved first part of it
differentiate the whole equation with respeact to x

you will get

3ax^2+2bx+c

find the maxima and minima

i.e for two turning points two distinct max,min must exist

Discriminant>0

(2b)^2-12ac>0
4*b^2 > 12 ac
b^2 >3 ac...prooved
 
Last edited:
second part

3ax^2+2bx+c=0 was the equation i already mentioned
one answer was 1/2,second 3/2
adding them 2
multiplying them we get 3/4

2=-2b/3a
-b=3a

3/4=c/3a
4c=9a

substituting value of function in the function equation we get

a+2b+4c+8d=8
27a+18b+12c+8d=-8

solve these equations simultaniously
for a,b,c,d

we get

d=-1
a=4
b=-12
c=9

if iam correct
 
Last edited:
I don't understand your last statement ..
please pardon...:smile:
 
Solved

Ah, I now understand it. I never thought of delta before
Thanks a lot
 

Similar threads

  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 1 ·
Replies
1
Views
1K
Replies
7
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 3 ·
Replies
3
Views
3K
Replies
24
Views
3K