# 4.7 Maxima/minima problems  (Page 7/10)

 Page 7 / 10

## Key concepts

• A critical point of the function $f\left(x,y\right)$ is any point $\left({x}_{0},{y}_{0}\right)$ where either ${f}_{x}\left({x}_{0},{y}_{0}\right)={f}_{y}\left({x}_{0},{y}_{0}\right)=0,$ or at least one of ${f}_{x}\left({x}_{0},{y}_{0}\right)$ and ${f}_{y}\left({x}_{0},{y}_{0}\right)$ do not exist.
• A saddle point is a point $\left({x}_{0},{y}_{0}\right)$ where ${f}_{x}\left({x}_{0},{y}_{0}\right)={f}_{y}\left({x}_{0},{y}_{0}\right)=0,$ but $\left({x}_{0},{y}_{0}\right)$ is neither a maximum nor a minimum at that point.
• To find extrema of functions of two variables, first find the critical points, then calculate the discriminant and apply the second derivative test.

## Key equations

• Discriminant
$D={f}_{xx}\left({x}_{0},{y}_{0}\right){f}_{yy}\left({x}_{0},{y}_{0}\right)-{\left({f}_{xy}\left({x}_{0},{y}_{0}\right)\right)}^{2}$

For the following exercises, find all critical points.

$f\left(x,y\right)=1+{x}^{2}+{y}^{2}$

$f\left(x,y\right)={\left(3x-2\right)}^{2}+{\left(y-4\right)}^{2}$

$\left(\frac{2}{3},4\right)$

$f\left(x,y\right)={x}^{4}+{y}^{4}-16xy$

$f\left(x,y\right)=15{x}^{3}-3xy+15{y}^{3}$

$\left(0,0\right)$ $\left(\frac{1}{15},\frac{1}{15}\right)$

For the following exercises, find the critical points of the function by using algebraic techniques (completing the square) or by examining the form of the equation. Verify your results using the partial derivatives test.

$f\left(x,y\right)=\sqrt{{x}^{2}+{y}^{2}+1}$

$f\left(x,y\right)=\text{−}{x}^{2}-5{y}^{2}+8x-10y-13$

Maximum at $\left(4,-1,8\right)$

$f\left(x,y\right)={x}^{2}+{y}^{2}+2x-6y+6$

$f\left(x,y\right)=\sqrt{{x}^{2}+{y}^{2}}+1$

Relative minimum at $\left(0,0,1\right)$

For the following exercises, use the second derivative test to identify any critical points and determine whether each critical point is a maximum, minimum, saddle point, or none of these.

$f\left(x,y\right)=\text{−}{x}^{3}+4xy-2{y}^{2}+1$

$f\left(x,y\right)={x}^{2}{y}^{2}$

The second derivative test fails. Since ${x}^{2}{y}^{2}>0$ for all x and y different from zero, and ${x}^{2}{y}^{2}=0$ when either x or y equals zero (or both), then the absolute minimum occurs at $\left(0,0\right).$

$f\left(x,y\right)={x}^{2}-6x+{y}^{2}+4y-8$

$f\left(x,y\right)=2xy+3x+4y$

$f\left(-2,-\frac{3}{2}\right)=-6$ is a saddle point.

$f\left(x,y\right)=8xy\left(x+y\right)+7$

$f\left(x,y\right)={x}^{2}+4xy+{y}^{2}$

$f\left(0,0\right)=0;$ $\left(0,0,0\right)$ is a saddle point.

$f\left(x,y\right)={x}^{3}+{y}^{3}-300x-75y-3$

$f\left(x,y\right)=9-{x}^{4}{y}^{4}$

$f\left(0,0\right)=9$ is a local maximum.

$f\left(x,y\right)=7{x}^{2}y+9x{y}^{2}$

$f\left(x,y\right)=3{x}^{2}-2xy+{y}^{2}-8y$

Relative minimum located at $\left(2,6\right).$

$f\left(x,y\right)=3{x}^{2}+2xy+{y}^{2}$

$f\left(x,y\right)={y}^{2}+xy+3y+2x+3$

$\left(1,-2\right)$ is a saddle point.

$f\left(x,y\right)={x}^{2}+xy+{y}^{2}-3x$

$f\left(x,y\right)={x}^{2}+2{y}^{2}-{x}^{2}y$

$\left(2,1\right)$ and $\left(-2,1\right)$ are saddle points; $\left(0,0\right)$ is a relative minimum.

$f\left(x,y\right)={x}^{2}+y-{e}^{y}$

$f\left(x,y\right)={e}^{\text{−}\left({x}^{2}+{y}^{2}+2x\right)}$

$\left(-1,0\right)$ is a relative maximum.

$f\left(x,y\right)={x}^{2}+xy+{y}^{2}-x-y+1$

$f\left(x,y\right)={x}^{2}+10xy+{y}^{2}$

$\left(0,0\right)$ is a saddle point.

$f\left(x,y\right)=\text{−}{x}^{2}-5{y}^{2}+10x-30y-62$

$f\left(x,y\right)=120x+120y-xy-{x}^{2}-{y}^{2}$

The relative maximum is at $\left(40,40\right).$

$f\left(x,y\right)=2{x}^{2}+2xy+{y}^{2}+2x-3$

$f\left(x,y\right)={x}^{2}+x-3xy+{y}^{3}-5$

$\left(\frac{1}{4},\frac{1}{2}\right)$ is a saddle point and $\left(1,1\right)$ is the relative minimum.

$f\left(x,y\right)=2xy{e}^{\text{−}{x}^{2}-{y}^{2}}$

For the following exercises, determine the extreme values and the saddle points. Use a CAS to graph the function.

[T] $f\left(x,y\right)=y{e}^{x}-{e}^{y}$

A saddle point is located at $\left(0,0\right).$

[T] $f\left(x,y\right)=x\phantom{\rule{0.2em}{0ex}}\text{sin}\left(y\right)$

[T] $f\left(x,y\right)=\text{sin}\left(x\right)\text{sin}\left(y\right),x\in \left(0,2\pi \right),y\in \left(0,2\pi \right)$

There is a saddle point at $\left(\pi ,\pi \right),$ local maxima at $\left(\frac{\pi }{2},\frac{\pi }{2}\right)\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}\left(\frac{3\pi }{2},\frac{3\pi }{2}\right),$ and local minima at $\left(\frac{\pi }{2},\frac{3\pi }{2}\right)\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}\left(\frac{3\pi }{2},\frac{\pi }{2}\right).$

Find the absolute extrema of the given function on the indicated closed and bounded set $R.$

$f\left(x,y\right)=xy-x-3y;$ $R$ is the triangular region with vertices $\left(0,0\right),\left(0,4\right),\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}\left(5,0\right).$

Find the absolute maximum and minimum values of $f\left(x,y\right)={x}^{2}+{y}^{2}-2y+1$ on the region $R=\left\{\left(x,y\right)|{x}^{2}+{y}^{2}\le 4\right\}.$

$\left(0,1,0\right)$ is the absolute minimum and $\left(0,-2,9\right)$ is the absolute maximum.

$f\left(x,y\right)={x}^{3}-3xy-{y}^{3}$ on $R=\left\{\left(x,y\right)\text{:}\phantom{\rule{0.2em}{0ex}}-2\le x\le 2,-2\le y\le 2\right\}$

$f\left(x,y\right)=\frac{-2y}{{x}^{2}+{y}^{2}+1}$ on $R=\left\{\left(x,y\right)\text{:}\phantom{\rule{0.2em}{0ex}}{x}^{2}+{y}^{2}\le 4\right\}$

There is an absolute minimum at $\left(0,1,-1\right)$ and an absolute maximum at $\left(0,-1,1\right).$

Find three positive numbers the sum of which is $27,$ such that the sum of their squares is as small as possible.

Find the points on the surface ${x}^{2}-yz=5$ that are closest to the origin.

$\left(\sqrt{5},0,0\right),\left(\text{−}\sqrt{5},0,0\right)$

Find the maximum volume of a rectangular box with three faces in the coordinate planes and a vertex in the first octant on the line $x+y+z=1.$

The sum of the length and the girth (perimeter of a cross-section) of a package carried by a delivery service cannot exceed $108$ in. Find the dimensions of the rectangular package of largest volume that can be sent.

$\text{18 by 36 by 18 in}.$

A cardboard box without a lid is to be made with a volume of $4$ ft 3 . Find the dimensions of the box that requires the least amount of cardboard.

Find the point on the surface $f\left(x,y\right)={x}^{2}+{y}^{2}+10$ nearest the plane $x+2y-z=0.$ Identify the point on the plane.

$\left(\frac{47}{24},\frac{47}{12},\frac{235}{24}\right)$

Find the point in the plane $2x-y+2z=16$ that is closest to the origin.

A company manufactures two types of athletic shoes: jogging shoes and cross-trainers. The total revenue from $x$ units of jogging shoes and $y$ units of cross-trainers is given by $R\left(x,y\right)=-5{x}^{2}-8{y}^{2}-2xy+42x+102y,$ where $x$ and $y$ are in thousands of units. Find the values of x and y to maximize the total revenue.

$x=3$ and $y=6$

A shipping company handles rectangular boxes provided the sum of the length, width, and height of the box does not exceed $96$ in. Find the dimensions of the box that meets this condition and has the largest volume.

Find the maximum volume of a cylindrical soda can such that the sum of its height and circumference is $120$ cm.

$V=\frac{64,000}{\pi }\approx 20,372$ cm 3

#### Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
can you provide the details of the parametric equations for the lines that defince doubly-ruled surfeces (huperbolids of one sheet and hyperbolic paraboloid). Can you explain each of the variables in the equations?
Radek Reply

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