MHB What Are the Maximum and Minimum Values of P Given the Equation?

  • Thread starter Thread starter anemone
  • Start date Start date
  • Tags Tags
    Maximum Minimum
AI Thread Summary
The discussion focuses on finding the minimum and maximum values of the expression P = (y - x) / (x + 8y) under the constraint defined by the equation y^2(6 - x^2) - xy - 1 = 0. Participants are encouraged to share their methods for solving the problem, indicating a collaborative approach to problem-solving. A hint is provided affirming the correctness of previous answers, suggesting that the solutions may already be established. The conversation emphasizes the importance of different solving techniques and encourages sharing insights. Overall, the thread highlights a mathematical exploration of optimization within a specific equation.
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
Find the minimum and maximum of $P=\dfrac{y−x}{x+8y}$ for all real $x$ and $y$ that satisfy the equation $y^2(6-x^2)-xy-1=0$.
 
Mathematics news on Phys.org
Hint:

If you're fond of using the AM-GM inequality method (when applicable) to solve for optimization problem, then you could try to express $P$ in terms of $xy$.
 
anemone said:
Hint:

If you're fond of using the AM-GM inequality method (when applicable) to solve for optimization problem, then you could try to express $P$ in terms of $xy$.
$max(P)=\dfrac {7}{2},\,\, xy=-1$
$min(P)=\dfrac {-1}{10},\, xy=1$
 
Last edited:
Yes, those are the correct answers, Albert. If your method of solving is different than mine, I hope you could post it to share it with me and MHB. Thanks. :)

Here is my solution:
$\begin{align*}P&=\dfrac{y−x}{x+8y}\\&=\dfrac{y(y−x)}{y(x+8y)}\\&=\dfrac{y^2−xy}{xy+8y^2}\\&=\dfrac{\dfrac{x^2y^2+xy+1}{6}−xy}{xy+8\left(\dfrac{x^2y^2+xy+1}{6}\right)}\\&=\dfrac{x^2y^2+xy+1-6xy}{6xy+8x^2y^2+8xy+8}\\&=\dfrac{x^2y^2-5xy+1}{8x^2y^2+14xy+8}\\&=\dfrac{x^2y^2-5xy+1}{8(x^2y^2+\dfrac{14xy}{8}+1)}\\&=\dfrac{1}{8}\left(1-\dfrac{27xy}{4(x^2y^2+\dfrac{14xy}{8}+1)}\right)\\&=\dfrac{1}{8}\left(1-\dfrac{27}{4(\dfrac{14}{8}+xy+\dfrac{1}{xy})}\right)\end{align*}$

If $xy\gt 0$, we get $P_{\text{minimum}}=\dfrac{1}{8}\left(1-\dfrac{27}{4(\dfrac{14}{8}+2)}\right)=-\dfrac{1}{10}$.

If $xy\lt 0$, then we need to rewrite $P$ such that it takes the form:

$\begin{align*}P&=\dfrac{1}{8}\left(1-\dfrac{27}{4(\dfrac{14}{8}+xy+\dfrac{1}{xy})}\right)\\&=\dfrac{1}{8}\left(1-\dfrac{27}{4(\dfrac{14}{8}-|xy|-\dfrac{1}{|xy|})}\right)\\&=\dfrac{1}{8}\left(1+\dfrac{27}{4\left(|xy|+\dfrac{1}{|xy|}-\dfrac{14}{8}\right)}\right)\end{align*}$

Thus $P_{\text{maximum}}=\dfrac{1}{8}\left(1+\dfrac{27}{4\left(2-\dfrac{14}{8}\right)}\right)=\dfrac{7}{2}$.
 
Albert said:
$max(P)=\dfrac {7}{2},\,\, xy=-1$
$min(P)=\dfrac {-1}{10},\, xy=1$
my solution:
let $k=xy$
$p=\dfrac {k^2-5k+1}{8k^2+14k+8}$ (up to now the solution is similar to anemone's method)
if $k>0 $ then :
$P\geq \dfrac{2k-5k}{8k^2+14k+8}=\dfrac {-3k}{8k^2+14k+8}=\dfrac {-1}{10}$ equality holds when $k=xy=1$
if $k<0 $ then :
$P\leq \dfrac{k^2-5k+1}{14k-16k}=\dfrac {k^2-5k+1}{-2k}=\dfrac {7}{2}$ equality holds when $k=xy=-1$
 
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
For original Zeta function, ζ(s)=1+1/2^s+1/3^s+1/4^s+... =1+e^(-sln2)+e^(-sln3)+e^(-sln4)+... , Re(s)>1 If we regards it as some function got from Laplace transformation, and let this real function be ζ(x), that means L[ζ(x)]=ζ(s), then: ζ(x)=L^-1[ζ(s)]=δ(x)+δ(x-ln2)+δ(x-ln3)+δ(x-ln4)+... , this represents a series of Dirac delta functions at the points of x=0, ln2, ln3, ln4, ... , It may be still difficult to understand what ζ(x) means, but once it is integrated, the truth is clear...
Back
Top