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USAMO 1973 #4
*August 19, 2009*

*Posted by lumixedia in algebra, Problem-solving.*

Tags: algebra, contest math, olympiad math, USAMO, USAMO 1973

7 comments

Tags: algebra, contest math, olympiad math, USAMO, USAMO 1973

7 comments

A fairly straightforward algebra problem. Could appear on a modern AMC-12, though the decoy answers would have to be carefully written.

**USAMO 1973 #4.** Determine all the roots, real or complex, of the system of simultaneous equations

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USAMO 1973 #3
*August 17, 2009*

*Posted by lumixedia in combinatorics, Problem-solving.*

Tags: combinatorics, contest math, olympiad math, USAMO, USAMO 1973

1 comment so far

Tags: combinatorics, contest math, olympiad math, USAMO, USAMO 1973

1 comment so far

**USAMO 1973 #3.** Three distinct vertices are chosen at random from the vertices of a given regular polygon of sides. If all such choices are equally likely, what is the probability that the center of the given polygon lies in the interior of the triangle determined by the three chosen random points? (more…)

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USAMO 1973 #2
*August 11, 2009*

*Posted by lumixedia in Problem-solving.*

Tags: algebra, contest math, number theory, olympiad math, USAMO, USAMO 1973

3 comments

Tags: algebra, contest math, number theory, olympiad math, USAMO, USAMO 1973

3 comments

**USAMO 1973 #2**. Let and denote two sequences of integers defined as follows:

Thus, the first few terms of the sequence are:

Prove that, except for “1”, there is no term which occurs in both sequences. (more…)

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USAMO 1973 #1
*August 7, 2009*

*Posted by lumixedia in Problem-solving.*

Tags: contest math, geometry, olympiad math, USAMO, USAMO 1973

2 comments

Tags: contest math, geometry, olympiad math, USAMO, USAMO 1973

2 comments

**USAMO 1973 #1.** Two points, and , lie in the interior of a regular tetrahedron . Prove that angle . (more…)

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USAMO 1972 #5
*August 4, 2009*

*Posted by lumixedia in Problem-solving.*

Tags: contest math, geometry, olympiad math, USAMO, USAMO 1972

2 comments

Tags: contest math, geometry, olympiad math, USAMO, USAMO 1972

2 comments

**USAMO 1972 #5.** A given convex pentagon has the property that the area of each of the five triangles , , , , is unity. Show that every non-congruent pentagon with the above property has the same area, and that, furthermore, there are an infinite number of such non-congruent pentagons. (more…)

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USAMO 1972 #4
*July 26, 2009*

*Posted by lumixedia in Problem-solving.*

Tags: algebra, contest math, olympiad math, USAMO, USAMO 1972

3 comments

Tags: algebra, contest math, olympiad math, USAMO, USAMO 1972

3 comments

**USAMO 1972 #4.** Let denote a non-negative rational number. Determine a fixed set of integers , , , , , such that, for *every* choice of ,

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USAMO 1972 #2, #3
*July 21, 2009*

*Posted by lumixedia in Problem-solving.*

Tags: combinatorics, contest math, geometry, olympiad math, USAMO, USAMO 1972

3 comments

Tags: combinatorics, contest math, geometry, olympiad math, USAMO, USAMO 1972

3 comments

I think I might as well just start going through the USAMOs in chronological/numerical order.

**USAMO 1972 #2.** A given tetrahedron is isosceles, that is , , . Show that the faces of the tetrahedron are acute-angled triangles. (more…)

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USAMO 2009 #5
*July 19, 2009*

*Posted by Damien Jiang in Problem-solving, Uncategorized.*

Tags: geometry, olympiad math

1 comment so far

Tags: geometry, olympiad math

1 comment so far

I like Olympiad geometry. Therefore, I will give my solution to this year’s USAMO #5; I was rather happy with my solution.

5. Trapezoid , with , is inscribed in circle and point lies inside triangle . Rays and meet again at points and , respectively. Let the line through parallel to intersects and at points and , respectively. Prove that quadrilateral is cyclic if and only if bisects .

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USAMO 1972 #1
*July 18, 2009*

*Posted by lumixedia in Problem-solving.*

Tags: contest math, number theory, olympiad math, USAMO, USAMO 1972

9 comments

Tags: contest math, number theory, olympiad math, USAMO, USAMO 1972

9 comments

My first post was going to be an introduction to combinatorial game theory, but putting that together would have been rather more complicated than grabbing some USAMO problem and putting up my solution, so of course I chose the path of less resistance. The intro to game theory will come eventually, but in the meantime, here’s the first USAMO problem ever:

**USAMO 1972 # 1.** The symbols and denote the greatest common divisor and the least common multiple, respectively, of the positive integers . For example, and . Prove that

Here is, based on my first instinct when seeing this problem… (more…)