Maxima and minima properties problem

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Homework Help Overview

The problem involves determining the value of a constant difference in an arithmetic progression (A.P.) that maximizes the product of certain terms. The context centers around properties of maxima and minima in relation to the arithmetic sequence.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss applying the Arithmetic Mean-Geometric Mean inequality (AM-GM) to relate the terms of the A.P. and question the implications of their findings regarding maximum values. There is also mention of differentiating to find critical points related to maxima and minima.

Discussion Status

Some participants have provided hints and alternative approaches, including the use of differentiation to explore maxima and minima properties. There is an ongoing examination of the assumptions made regarding the relationships between the left-hand side (LHS) and right-hand side (RHS) of the inequality.

Contextual Notes

Participants are navigating potential misunderstandings about the implications of their calculations and the relationship between the LHS and RHS in the context of maximizing the product of the terms in the A.P.

zorro
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Homework Statement



Let An be the nth term of an A.P. and if A7 = 15, then the value of the c.d. that would make A3 x A7 x A12 greatest is :

1)9
2)9/4
3)3/8
4)18

Homework Equations





The Attempt at a Solution



Applying AM>=GM

A3+A7+A12/3 >= (A3 x A7 x A12)^1/3

given that A+6d=15
therefore 3A+19d=45

The previous expression reduces to 45+d/3 >= (A3 x A7 x A12)^1/3
cubing the inequality.
(45+d/3)^3 >= (A3 x A7 x A12)

From the choices, 18 will make RHS greatest. But that is not correct!
Any help appreciated.
 
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nice one.. it is.. sure some iit problem
I believe your 2nd last statement is wrong because 18 makes the LHS greatest not RHS and RHS has to be lesser than LHS.

Actually i tried a different approach and i'll give you a hint.
Use the maxima and minima properties and differentiate finding out the values of d. (you will get 2 values ..though one will be eliminated)
 


Yes, 18 makes the LHS greatest. Since RHS is </= LHS, greater LHS implies greater RHS.
Any way I got the answer by your method :smile:
I just wanted to know what was wrong in this.
 


Your method doesn't exactly give you the answer... it just tells you when LHS is greatest and gives no info about RHS(which is what you want). Though this approach may be modified to get the answer.
(I will try that out)
 

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