×

# Symmetric Inequality Problem

I am struggling with understanding https://nrich.maths.org/251.

To restate the problem,

If $$x$$, $$y$$ and $$z$$ are real numbers such that: $$\begin{cases}x+y+z=5 \\ xy+yz+zx=3\end{cases}$$ , What is the largest value that any one of these numbers can have?

In particular, I do not understand the first solution given, and while the second I am getting a grip with (creates a quadratic uses the discriminant inequality since $$x$$, $$y$$ and $$z$$ are real numbers), would like to ask whether any classical inequalities can be used here, as I would be personally more satisfied with this.

The problem I had with applying inequalities I knew was that $$x$$, $$y$$ and $$z$$ could be any real numbers, not just positive.

Any help/discussion would be much appreciated!

Note by Arthur Conmy
7 months ago

MarkdownAppears as
*italics* or _italics_ italics
**bold** or __bold__ bold
- bulleted- list
• bulleted
• list
1. numbered2. list
1. numbered
2. list
Note: you must add a full line of space before and after lists for them to show up correctly
paragraph 1paragraph 2

paragraph 1

paragraph 2

[example link](https://brilliant.org)example link
> This is a quote
This is a quote
    # I indented these lines
# 4 spaces, and now they show
# up as a code block.

print "hello world"
# I indented these lines
# 4 spaces, and now they show
# up as a code block.

print "hello world"
MathAppears as
Remember to wrap math in $$...$$ or $...$ to ensure proper formatting.
2 \times 3 $$2 \times 3$$
2^{34} $$2^{34}$$
a_{i-1} $$a_{i-1}$$
\frac{2}{3} $$\frac{2}{3}$$
\sqrt{2} $$\sqrt{2}$$
\sum_{i=1}^3 $$\sum_{i=1}^3$$
\sin \theta $$\sin \theta$$
\boxed{123} $$\boxed{123}$$

Sort by:

i m getting $$\frac{13}{3}$$ as maximum value and -1 as minimum value

- 7 months ago

Yep same here! What was your method?

- 7 months ago

but u said u are getting $$\frac{13}{2}$$ . I used the same method which i referred to in the link i gave.

- 7 months ago

Oops! Sorry I am getting $$\frac{13}{3}$$. I actually did not check the link out. Let me check it now

- 7 months ago

- 7 months ago

Cauchy Schwarz inequality

- 7 months ago

same method as mine then? or in a different way?

- 7 months ago

Hey buddy is the answer $$\frac{13}{2}$$?

- 7 months ago

okay ! great

- 7 months ago

actually u have put a dot by mistake in front of 251

- 7 months ago

The link u have referred is not opening, it says the page not found.However, u can see my solution to this problem : https://brilliant.org/problems/almost-vietas/#!/solution-comments/171217/

- 7 months ago

- 7 months ago

Fixed btw

- 7 months ago

yup

- 7 months ago