## Secondary and Higher Secondary Marathon

For students of class 11-12 (age 16+)
Posts: 1016
Joined: Tue Nov 22, 2011 7:49 pm
Location: 127.0.0.1
Contact:

### Secondary and Higher Secondary Marathon

Here we'll talk about national level problems.
Rules:
1. You can post any 'math-problem' from anywhere if you are sure it has a solution. Also you should give the source.(Like book name, link or self-made)
2. If a problem remains unsolved for two days, the proposer must post the solution (for self-made problems) or the official solution will be posted. (for contest problems)
3. Anyone can post a new problem iff the previous problem has been solved already.
4. Don't forget to type the problem number.
Welcome to BdMO Online Forum. Check out Forum Guides & Rules

SANZEED
Posts: 550
Joined: Wed Dec 28, 2011 6:45 pm

### Re: Secondary and Higher Secondary Marathon

Problem 1
Prove that the expression
$m^5+3m^4n-5m^3n^2-15m^2n^3+4mn^4+12n^5$
can't have the value $33$ regardless of what integers are substituted for $m,n$.

Source:The USSR Math Olympiad Problem Book.
$\color{blue}{\textit{To}} \color{red}{\textit{ problems }} \color{blue}{\textit{I am encountering with-}} \color{green}{\textit{AVADA KEDAVRA!}}$

Fahim Shahriar
Posts: 138
Joined: Sun Dec 18, 2011 12:53 pm

### Re: Secondary and Higher Secondary Marathon

Problem 1 Solution:
Simplifying the expression we get,

$m^5+3m^4n-5m^3n^2-15m^2n^3+4mn^4+12n^5$
$=(m+n)(m-n)(m+2n)(m-2n)(m+3n)$

It has five factors. On the other hand, 33 has only two factors. If one factor or the product of two factors of the expression is 33, the other factors will be 1. But then we will get $m=n$ and a factor will be 0.
Therefore, the expression can't have the value 33.
Name: Fahim Shahriar Shakkhor
Notre Dame College

Fahim Shahriar
Posts: 138
Joined: Sun Dec 18, 2011 12:53 pm

### Re: Secondary and Higher Secondary Marathon

Problem 2
$ABCDEFGHI$ is a regular nonagon . show that $BG = BC + BD$
Name: Fahim Shahriar Shakkhor
Notre Dame College

SANZEED
Posts: 550
Joined: Wed Dec 28, 2011 6:45 pm

### Re: Secondary and Higher Secondary Marathon

For Problem $1$
I think I should indicate to a few facts here: $33$ can be expressed as a product of $4$ different factors.
$33=11\cdot (-3)\cdot 1\cdot (-1)$
$33=(-11)\cdot 3\cdot 1\cdot (-1)$. Secondly,my proof is as follows: if $n\neq 0$,then none of the factors are equal. If $n=0$, then the expression becomes $m^5$,which can't be equal to $33$.
$\color{blue}{\textit{To}} \color{red}{\textit{ problems }} \color{blue}{\textit{I am encountering with-}} \color{green}{\textit{AVADA KEDAVRA!}}$

Posts: 1016
Joined: Tue Nov 22, 2011 7:49 pm
Location: 127.0.0.1
Contact:

### Re: Secondary and Higher Secondary Marathon

Hints for prob 2:
WLOG we may assume the nonagon inscribed in the unit circle. Now use complex numbers.
Welcome to BdMO Online Forum. Check out Forum Guides & Rules

nafistiham
Posts: 829
Joined: Mon Oct 17, 2011 3:56 pm
Location: 24.758613,90.400161
Contact:

### Re: Secondary and Higher Secondary Marathon

Phlembac Adib Hasan wrote:Hints for prob 2:
WLOG we may assume the nonagon inscribed in the unit circle. Now use complex numbers.
Yes, unit circles is the trick. But, we may not to have to go to that much complexity. just, simple trigonometry can serve us the solution.

a little trigonometric hint.
$\sum_{k=0}^{n-1}e^{\frac{2 \pi i k}{n}}=0$
Using $L^AT_EX$ and following the rules of the forum are very easy but really important, too.Please co-operate.

Fahim Shahriar
Posts: 138
Joined: Sun Dec 18, 2011 12:53 pm

### Re: Secondary and Higher Secondary Marathon

Problem 2 Solution

Someone post problem 3.
Name: Fahim Shahriar Shakkhor
Notre Dame College

Tahmid Hasan
Posts: 665
Joined: Thu Dec 09, 2010 5:34 pm

### Re: Secondary and Higher Secondary Marathon

Problem 3: Circles $W_1,W_2$ meet at $D$ and $P$. $A$ and $B$ are on $W_1,W_2$ respectively, such that $AB$ is tangent to $W_1$ and $W_2$. Suppose $D$ is closer than $P$ to the line $AB$. $AD$ meet circle $W_2$ for second time at $C$. Let $M$ be the midpoint of $BC$. Prove that $\angle{DPM}=\angle{BDC}$.
Source: Iran NMO-2010-3.
Note: High-school textbook theorems are enough, so I thought it would be okay to post here.
বড় ভালবাসি তোমায়,মা

SANZEED
Posts: 550
Joined: Wed Dec 28, 2011 6:45 pm

### Re: Secondary and Higher Secondary Marathon

Problem 3
Since $\angle BPM$ is common,so we just need to show that $\angle DPM=\angle CPM$. Let the extension of $PD$ meet $AB$ at $E$. Now,since $PE$ is the radical axis of the two circles,and $AB$ is tangent to the circles,we must have $EA^2=EB^2\Rightarrow EA=EB$. So $EM\parallel AC$. Thus, $\angle EPB=\angle DPB=\angle DCB=\angle EMB$
and $E,B,M,P$ are con cyclic. Again, $\angle EBD=\angle BCD=\angle BPD$.
Now,in cyclic quad $EBMP$, we have that $\angle EBM+\angle EPM=180^{\circ}$ and in cyclic quad $BDPC$ we have $\angle DBC+\angle DPC=180^{\circ}$.
So, $\angle BPD=\angle EBD=\angle EBM-\angle DBC=180^{\circ}-\angle EPM-180^{\circ}+\angle DPC=\angle CPM$, which completes the proof.

P.S.Someone post problem $4$ please.
Attachments Marathon second 3.png (123.49 KiB) Viewed 4777 times
$\color{blue}{\textit{To}} \color{red}{\textit{ problems }} \color{blue}{\textit{I am encountering with-}} \color{green}{\textit{AVADA KEDAVRA!}}$