Found problems: 6530
2020 India National Olympiad, 4
Let $n \geqslant 2$ be an integer and let $1<a_1 \le a_2 \le \dots \le a_n$ be $n$ real numbers such that $a_1+a_2+\dots+a_n=2n$. Prove that$$a_1a_2\dots a_{n-1}+a_1a_2\dots a_{n-2}+\dots+a_1a_2+a_1+2 \leqslant a_1a_2\dots a_n.$$
[i]Proposed by Kapil Pause[/i]
2014 India Regional Mathematical Olympiad, 6
For any natural number, let $S(n)$ denote sum of digits of $n$. Find the number of $3$ digit numbers for which $S(S(n)) = 2$.
2003 Federal Competition For Advanced Students, Part 2, 2
Let $a, b, c$ be nonzero real numbers for which there exist $\alpha, \beta, \gamma \in\{-1, 1\}$ with $\alpha a + \beta b + \gamma c = 0$. What is the smallest possible value of
\[\left( \frac{a^3+b^3+c^3}{abc}\right)^2 ?\]
2009 Petru Moroșan-Trident, 2
If $ a,b,c>0$ and $ a\plus{}b\plus{}c\equal{}3$ prove that:
$ \frac{{a^3 \left( {a \plus{} b} \right)}}{{a^2 \plus{} ab \plus{} b^2 }} \plus{} \frac{{b^3 \left( {b \plus{} c} \right)}}{{b^2 \plus{} bc \plus{} c^2 }} \plus{} \frac{{c^3 \left( {c \plus{} a} \right)}}{{c^2 \plus{} ac \plus{} a^2 }} \ge 2$
P.s.
I dont know if it has been posted before.
2009 Singapore MO Open, 4
find largest constant C st
$\sum_{i=1}^{4} (x_i+1/x_i)^3 \geq C$
for all positive real numbers $x_1,..,x_4$ st
$x_1^3+x_3^3+3x_1x_3=x_2+x_4=1$
2007 Turkey Team Selection Test, 3
Let $a, b, c$ be positive reals such that their sum is $1$. Prove that \[\frac{1}{ab+2c^{2}+2c}+\frac{1}{bc+2a^{2}+2a}+\frac{1}{ac+2b^{2}+2b}\geq \frac{1}{ab+bc+ac}.\]
1971 Poland - Second Round, 5
Given the set of numbers $ \{1, 2, 3, \ldots, 100\} $. From this set, create 10 pairwise disjoint subsets $ N_i = \{a_{i,1}, a_{i,2}, ... a_{i,10} $ ($ i = 1, 2, \ldots, 10 $ ) so that the sum of the products
$$
\sum_{i=10}^{10}\prod_{j=1}^{10} a_{i,j}
$$
was the biggest.
1998 Singapore Team Selection Test, 2
Let $ a_1\geq \cdots \geq a_n \geq a_{n \plus{} 1} \equal{} 0$ be real numbers. Show that
\[ \sqrt {\sum_{k \equal{} 1}^n a_k} \leq \sum_{k \equal{} 1}^n \sqrt k (\sqrt {a_k} \minus{} \sqrt {a_{k \plus{} 1}}).
\]
[i]Proposed by Romania[/i]
1992 China Team Selection Test, 3
For any $n,T \geq 2, n, T \in \mathbb{N}$, find all $a \in \mathbb{N}$ such that $\forall a_i > 0, i = 1, 2, \ldots, n$, we have
\[\sum^n_{k=1} \frac{a \cdot k + \frac{a^2}{4}}{S_k} < T^2 \cdot \sum^n_{k=1} \frac{1}{a_k},\] where $S_k = \sum^k_{i=1} a_i.$
2002 Czech-Polish-Slovak Match, 2
A triangle $ABC$ has sides $BC = a, CA = b, AB = c$ with $a < b < c$ and area $S$. Determine the largest number $u$ and the least number $v$ such that, for every point $P$ inside $\triangle ABC$, the inequality $u \le PD + PE + PF \le v$ holds, where $D,E, F$ are the intersection points of $AP,BP,CP$ with the opposite sides.
2013 Hong kong National Olympiad, 1
Let $a,b,c$ be positive real numbers such that $ab+bc+ca=1$. Prove that
\[\sqrt[4]{\frac{\sqrt{3}}{a}+6\sqrt{3}b}+\sqrt[4]{\frac{\sqrt{3}}{b}+6\sqrt{3}c}+\sqrt[4]{\frac{\sqrt{3}}{c}+6\sqrt{3}a}\le\frac{1}{abc}\]
When does inequality hold?
2006 IMC, 5
Let $a, b, c, d$ three strictly positive real numbers such that \[a^{2}+b^{2}+c^{2}=d^{2}+e^{2},\] \[a^{4}+b^{4}+c^{4}=d^{4}+e^{4}.\] Compare \[a^{3}+b^{3}+c^{3}\] with \[d^{3}+e^{3},\]
2019 Jozsef Wildt International Math Competition, W. 38
Let $a$, $b$, $c$ be the sides of an acute triangle $\triangle ABC$ , then for any $x, y, z \geq 0$, such that $xy+yz+zx=1$ holds inequality:$$a^2x + b^2y + c^2z \geq 4F$$ where $F$ is the area of the triangle $\triangle ABC$
VI Soros Olympiad 1999 - 2000 (Russia), 11.9
Find the largest $c$ such that for any $\lambda \ge 1$ there is an a that satisfies the inequality
$$\sin a + \sin (a\lambda ) \ge c.$$
1975 All Soviet Union Mathematical Olympiad, 212
Prove that for all the positive numbers $a,b,c$ the following inequality is valid:
$$a^3+b^3+c^3+3abc>ab(a+b)+bc(b+c)+ac(a+c)$$
1995 Romania Team Selection Test, 1
Let $a_1, a_2,...., a_n$ be distinct positive integers.
Prove that $(a_1^5 + ...+ a_n^5) + (a_1^7 + ...+ a_n^7) \ge 2(a_1^3 + ...+ a_n^3)^2$ and find the cases of equality.
2018 Junior Balkan Team Selection Tests - Moldova, 3
Let $a,b,c \in\mathbb{R^*_+}$.Prove the inequality $\frac{a^2+4}{b+c}+\frac{b^2+9}{c+a}+\frac{c^2+16}{a+b}\ge9$.
VMEO III 2006 Shortlist, N13
Prove the following two inequalities:
1) If $n > 49$, then exist positive integers $a, b > 1$ such that $a+b=n$ and $$\frac{\phi (a)}{a}+\frac{\phi (b)}{b}<1$$
2) If $n > 4$, then exist integer integers $a, b > 1$ such that $a+b=n$ and $$\frac{\phi (a)}{a}+\frac{\phi (b)}{b}>1$$
1959 Czech and Slovak Olympiad III A, 2
Let $a, b, c$ be real numbers such that $a+b+c > 0$, $ab+bc+ca > 0$, $abc > 0$. Show that $a, b, c$ are all positive.
1996 IMO Shortlist, 8
Let $ ABCD$ be a convex quadrilateral, and let $ R_A, R_B, R_C, R_D$ denote the circumradii of the triangles $ DAB, ABC, BCD, CDA,$ respectively. Prove that $ R_A \plus{} R_C > R_B \plus{} R_D$ if and only if $ \angle A \plus{} \angle C > \angle B \plus{} \angle D.$
2013 Math Prize For Girls Problems, 20
Let $a_0$, $a_1$, $a_2$, $\dots$ be an infinite sequence of real numbers such that $a_0 = \frac{4}{5}$ and
\[
a_{n} = 2 a_{n-1}^2 - 1
\]
for every positive integer $n$. Let $c$ be the smallest number such that for every positive integer $n$, the product of the first $n$ terms satisfies the inequality
\[
a_0 a_1 \dots a_{n - 1} \le \frac{c}{2^n}.
\]
What is the value of $100c$, rounded to the nearest integer?
2005 Hong kong National Olympiad, 4
Let $a,b,c,d$ be positive real numbers such that $a+b+c+d=1$. Prove that\[ 6(a^3+b^3+c^3+d^3)\ge(a^2+b^2+c^2+d^2)+\frac{1}{8} \]
2009 Indonesia TST, 3
Let $ x,y,z$ be real numbers. Find the minimum value of $ x^2\plus{}y^2\plus{}z^2$ if $ x^3\plus{}y^3\plus{}z^3\minus{}3xyz\equal{}1$.
1982 Spain Mathematical Olympiad, 4
Determine a polynomial of non-negative real coefficients that satisfies the following two conditions:
$$p(0) = 0, p(|z|) \le x^4 + y^4,$$
being $|z|$ the module of the complex number $z = x + iy$ .
2016 Ukraine Team Selection Test, 6
Let $n$ be a fixed positive integer. Find the maximum possible value of \[ \sum_{1 \le r < s \le 2n} (s-r-n)x_rx_s, \] where $-1 \le x_i \le 1$ for all $i = 1, \cdots , 2n$.