# Algebra Set

Hi! This week's set consists in three problems in algebra that seem nice. I'm using the word "seem" because I've been working on them and haven't been able to solve them. Anyway, enjoy the problems, and, if you find them nice and/or fun, please reshare the note, so more people will be able to see the problems and try to tackle these hard problems. Just as a reference, problem 1 was taken from the Colombian book "Un recorrido por el álgebra" and problems 2 and 3 were taken from the first Costa Rican IMO TST 2014.

1. Find all functions $f:\mathbb{Z}^+\rightarrow\mathbb{R}$ such that $f(n+m)+f(n-m)=f(3n)$ for every $n,m\in{\mathbb{Z}^+}, n>m$.

2. Find all real solutions $(x_1,...,x_n)$ of the equation $(1-x_1)^2+(x_1-x_2)^2+...+(x_{n-1}-x_n)^2+x_n^2=\dfrac{1}{n+1}$

3. Let $x, y$ be real numbers such that $y>0$ and $7x^2+3xy+3y^2=1$. Show that $\dfrac{x^2+y^2}{y}\ge{\dfrac{1}{2}}$ Note by José Marín Guzmán
6 years, 4 months ago

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For the second one:

Using the Cauchy-Schwarz Inequality we can write, $\displaystyle \begin{array}{c}\\ \bigg( (1-x_1)^2 + (x_1 - x_2 )^2 +\dotsb + (x_{n-1} - x_n)^2 + x_n^2 \bigg) \bigg( 1^2 + 1^2 + \dotsb + 1^2 + 1^2 \bigg) \geq & \\ \bigg( (1-x_1) + (x_1 - x_2 ) +\dotsb + (x_{n-1} - x_n)+ x_n \bigg)^2 & \\ \end{array}$

$\displaystyle \Rightarrow \Big( (1-x_1)^2 + (x_1 - x_2 )^2 +\dotsb + (x_{n-1} - x_n)^2 + x_n^2 \Big) (n+1) \geq ( 1)^2$ $\displaystyle \Rightarrow (1-x_1)^2 + (x_1 - x_2 )^2 +\dotsb + (x_{n-1} - x_n)^2 + x_n^2 \geq \frac{1}{n+1}$

But it is given that the equality condition holds true. Therefore, from the condition of equality in Cauchy-Schwarz, we obtain,
$\displaystyle \frac{1-x_1}{1} = \frac{x_1 - x_2}{1} = \dots = \frac{x_{n-1} - x_n}{1} = \frac{x_n}{1}$ $\displaystyle \Rightarrow x_{n-1} = 2x_n \text{ , } x_{n-2} = 3x_n ,\dotsc, \text{ } x_2 = (n-1)x_n \text{ and } x_1 = nx_n$.
Also, $\displaystyle 1 = x_1 + x_n \Rightarrow (n+1)x_n = 1 \Rightarrow x_n = \frac{1}{n+1}$.

Hence, $\displaystyle x_i = \frac{n+1-i}{n+1} \text{ ; } i = 1,2,3, \dotsc , n-1, n$

- 6 years, 4 months ago

May I know how you arrive at a conclusion for $x_{i}$?

- 6 years, 4 months ago

From third last line, $x_{i} = (n+1-i)x_{n}$ and $x_{n} = \frac{1}{n+1}$.

- 6 years, 4 months ago

Thanks, I see it now :)

- 6 years, 4 months ago

I know of a "crude" solution to 3, which only uses basic knowledge of conic sections. It has a nice interpretation that way.

Staff - 6 years, 4 months ago

I'm afraid I don't know much about conic sections, but share the solution and I'll try to look up the stuff I don't understand :). Besides, it would be useful for other people who may know about conic sections.

- 6 years, 4 months ago

Show that the circle $2 x^2 + 2 y^2 - y = 0$ is contained within the ellipse $7x^2 + 3xy + 3y^2 = 1$. In fact, these conic sections are tangential at the point $( \frac{ 1}{5}, \frac{2}{5})$.

Hence, it follows that for any point on the ellipse which satisfy $y \geq 0$, we have

$2 x^2 + 2y^2 - y \geq 0 \Rightarrow \frac{ x^2+y^2} { y} \geq \frac{1}{2}.$

The tangential point is the equality case.

Staff - 6 years, 4 months ago

awesome man!!........... nice approach....... thanks:)

- 6 years, 4 months ago

For no.1 I have a weird start, this might helps.

Substituting $m \rightarrow 1, 2, 3, ..., n-1$ we get

$f(2n-i) + f(i) = f(3n) = f(2n-j) + f(j)$ for every $i,j = 1, 2, ..., n-1$

We see that if $a+b = c+d$, then $f(a)+f(b)=f(c)+f(d)$ for positive integers $a,b,c,d$.

I have no idea to do anything right meow. I suck at this thing XD

- 6 years, 4 months ago

Feel free to post ideas and solutions! :D

- 6 years, 4 months ago

For question $2$, does the solution only occur when $n=0$ with $x_{0}=1$?

- 6 years, 4 months ago