Olympiad problems

1987 Traian Lălescu, 1.4

Let $ ABCD $ be a regular tetahedron and $ M,N $ be middlepoints for $ AD, $ respectively, $ BC. $ Through a point $ P $ that is on segment $ MN, $ passes a plane perpendicular on $ MN, $ and meets the sides $ AB,AC,CD,BD $ of the tetahedron at $ E,F,G, $ respectively, $ H. $ [b]a)[/b] Prove that the perimeter of the quadrilateral $ EFGH $ doesn't depend on $ P. $ [b]b)[/b] Determine the maximum area of $ EFGH $ (depending on a side of the tetahedron).

2012 Gulf Math Olympiad, 4

Tags: calculus , integration , geometry , 3d geometry , sphere , modular arithmetic , number theory

Fawzi cuts a spherical cheese completely into (at least three) slices of equal thickness. He starts at one end, making successive parallel cuts, working through the cheese until the slicing is complete. The discs exposed by the first two cuts have integral areas. [list](i) Prove that all the discs that he cuts have integral areas. (ii) Prove that the original sphere had integral surface area if, and only if, the area of the second disc that he exposes is even.[/list]

1950 Moscow Mathematical Olympiad, 186

Tags: sphere , plane , 3d geometry , tangent , geometry

A spatial quadrilateral is circumscribed around a sphere. Prove that all the tangent points lie in one plane.

1976 USAMO, 4

Tags: geometry , 3d geometry , tetrahedron , pythagorean theorem

If the sum of the lengths of the six edges of a trirectangular tetrahedron $ PABC$ (i.e., $ \angle APB \equal{} \angle BPC \equal{} \angle CPA \equal{} 90^\circ$) is $ S$, determine its maximum volume.

1962 Bulgaria National Olympiad, Problem 3

Tags: geometry , 3d geometry

It is given a cube with sidelength $a$. Find the surface of the intersection of the cube with a plane, perpendicular to one of its diagonals and whose distance from the centre of the cube is equal to $h$.

2018 German National Olympiad, 2

Tags: geometry , 3d geometry , tetrahedron

We are given a tetrahedron with two edges of length $a$ and the remaining four edges of length $b$ where $a$ and $b$ are positive real numbers. What is the range of possible values for the ratio $v=a/b$?

2020 German National Olympiad, 6

Tags: 3d geometry , tetrahedron , isogonal conjugate , geometry

The insphere and the exsphere opposite to the vertex $D$ of a (not necessarily regular) tetrahedron $ABCD$ touch the face $ABC$ in the points $X$ and $Y$, respectively. Show that $\measuredangle XAB=\measuredangle CAY$.

1940 Moscow Mathematical Olympiad, 066

Tags: combinatorial geometry , geometry , cone , 3d geometry

* Given an infinite cone. The measure of its unfolding’s angle is equal to $\alpha$. A curve on the cone is represented on any unfolding by the union of line segments. Find the number of the curve’s self-intersections.

1971 Bulgaria National Olympiad, Problem 6

Tags: geometry , 3d geometry , pyramid , inequalities , geometric inequalities

In a triangular pyramid $SABC$ one of the plane angles with vertex $S$ is a right angle and the orthogonal projection of $S$ on the base plane $ABC$ coincides with the orthocenter of the triangle $ABC$. Let $SA=m$, $SB=n$, $SC=p$, $r$ is the inradius of $ABC$. $H$ is the height of the pyramid and $r_1,r_2,r_3$ are radii of the incircles of the intersections of the pyramid with the plane passing through $SA,SB,SC$ and the height of the pyramid. Prove that (a) $m^2+n^2+p^2\ge18r^2$; (b) $\frac{r_1}H,\frac{r_2}H,\frac{r_3}H$ are in the range $(0.4,0.5)$.

2011 Romania National Olympiad, 3

Tags: pyramid , angle , 3d geometry , geometry

Let $VABC$ be a regular triangular pyramid with base $ABC$, of center $O$. Points $I$ and $H$ are the center of the inscribed circle, respectively the orthocenter $\vartriangle VBC$. Knowing that $AH = 3 OI$, determine the measure of the angle between the lateral edge of the pyramid and the plane of the base.

1992 ITAMO, 1

Tags: combinatorial geometry , 3d geometry , cube , plane , geometry

A cube is divided into $27$ equal smaller cubes. A plane intersects the cube. Find the maximum possible number of smaller cubes the plane can intersect.

2004 Iran MO (3rd Round), 26

Tags: 3d geometry , sphere , combinatorial geometry , geometry

Finitely many points are given on the surface of a sphere, such that every four of them lie on the surface of open hemisphere. Prove that all points lie on the surface of an open hemisphere.

2010 Tournament Of Towns, 3

Tags: 3d geometry , octahedron , geometry

Is it possible to cover the surface of a regular octahedron by several regular hexagons without gaps and overlaps? (A regular octahedron has $6$ vertices, each face is an equilateral triangle, each vertex belongs to $4$ faces.)

2011 Federal Competition For Advanced Students, Part 1, 4

Tags: 3d geometry , tetrahedron , function , geometry

Inside or on the faces of a tetrahedron with five edges of length $2$ and one edge of lenght $1$, there is a point $P$ having distances $a, b, c, d$ to the four faces of the tetrahedron. Determine the locus of all points $P$ such that $a+b+c+d$ is minimal and the locus of all points $P$ such that $a+b+c+d$ is maximal.

1969 Czech and Slovak Olympiad III A, 6

Tags: geometry , 3d geometry , sphere , common tangent , tangent circle

A sphere with unit radius is given. Furthermore, circles $k_0,k_1,\ldots,k_n\ (n\ge3)$ of the same radius $r$ are given on the sphere. The circle $k_0$ is tangent to all other circles $k_i$ and every two circles $k_i,k_{i+1}$ are tangent for $i=1,\ldots,n$ (assuming $k_{n+1}=k_1$). a) Find relation between numbers $n,r.$ b) Determine for which $n$ the described situation can occur and compute the corresponding radius $r.$ (We say non-planar circles are tangent if they have only a single common point and their tangent lines in this point coincide.)

2007 AMC 12/AHSME, 25

Tags: geometry , 3d geometry , prism

Points $ A$, $ B$, $ C$, $ D$, and $ E$ are located in 3-dimensional space with $ AB \equal{} BC \equal{} CD \equal{} DE \equal{} EA \equal{} 2$ and $ \angle ABC \equal{} \angle CDE \equal{} \angle DEA \equal{} 90^\circ.$ The plane of $ \triangle ABC$ is parallel to $ \overline{DE}$. What is the area of $ \triangle BDE$? $ \textbf{(A)}\ \sqrt2 \qquad \textbf{(B)}\ \sqrt3 \qquad \textbf{(C)}\ 2 \qquad \textbf{(D)}\ \sqrt5 \qquad \textbf{(E)}\ \sqrt6$

1974 Spain Mathematical Olympiad, 1

Tags: geometry , 3d geometry , combinatorial geometry , dodecahedron

It is known that a regular dodecahedron is a regular polyhedron with $12$ faces of equal pentagons and concurring $3$ edges in each vertex. It is requested to calculate, reasonably, a) the number of vertices, b) the number of edges, c) the number of diagonals of all faces, d) the number of line segments determined for every two vertices, d) the number of diagonals of the dodecahedron.

1982 AMC 12/AHSME, 16

Tags: geometry , 3d geometry

A wooden cube has edges of length $3$ meters. Square holes, of side one meter, centered in each face are cut through to the opposite face. The edges of the holes are parallel to the edges of the cube. The entire surface area including the inside, in square meters, is $\textbf {(A) } 54 \qquad \textbf {(B) } 72 \qquad \textbf {(C) } 76 \qquad \textbf {(D) } 84\qquad \textbf {(E) } 86$

2008 Federal Competition For Advanced Students, Part 2, 2

Tags: geometry , 3d geometry , number theory

Which positive integers are missing in the sequence $ \left\{a_n\right\}$, with $ a_n \equal{} n \plus{} \left[\sqrt n\right] \plus{}\left[\sqrt [3]n\right]$ for all $ n \ge 1$? ($ \left[x\right]$ denotes the largest integer less than or equal to $ x$, i.e. $ g$ with $ g \le x < g \plus{} 1$.)

1994 Niels Henrik Abels Math Contest (Norwegian Math Olympiad) Round 2, 5

Tags: geometry , 3d geometry , rotation , symmetry

In how many ways can you color the six sides of a cube in black or white? (Do note that the cube is unchanged when rotated?) A. 7 B. 10 C. 20 D. 30 E. 36

1991 IMTS, 5

Tags: 3d geometry , tetrahedron , geometry

Show that it is impossible to dissect an arbitary tetrahedron into six parts by planes or portions thereof so that each of the parts has a plane of symmetry.

1949 Putnam, A4

Tags: geometry , 3d geometry , tetrahedron

Given that $P$ is a point inside a tetrahedron with vertices at $A, B, C$ and $D$, such that the sum of the distances $PA+PB+PC+PD$ is a minimum, show that the two angles $\angle APB$ and $\angle CPD$ are equal and are bisected by the same straight line. What other pair of angles must be equal?

1995 All-Russian Olympiad Regional Round, 10.7

Tags: combinatorics , combinatorial geometry , 3d geometry , cube , geometry

$N^3$ unit cubes are made into beads by drilling a hole through them along a diagonal, put on a string and binded. Thus the cubes can move freely in space as long as the vertices of two neighboring cubes (including the first and last one) are touching. For which $N$ is it possible to build a cube of edge $N$ using these cubes?

1997 All-Russian Olympiad Regional Round, 11.7

Tags: geometry , 3d geometry , pyramid , combinatorics , combinatorial geometry , angle

Are there convex $n$-gonal ($n \ge 4$) and triangular pyramids such that the four trihedral angles of the $n$-gonal pyramid are equal trihedral angles of a triangular pyramid? [hide=original wording] Существуют ли выпуклая n-угольная (n>= 4) и треугольная пирамиды такие, что четыре трехгранных угла n-угольной пирамиды равны трехгранным углам треугольной пирамиды?[/hide]

2000 Mongolian Mathematical Olympiad, Problem 3

Tags: 3d geometry , combinatorics , geometry

A cube of side $n$ is cut into $n^3$ unit cubes, and m of these cubes are marked so that the centers of any three marked cubes do not form a right-angled triangle with legs parallel to sides of the cube. Find the maximum possible value of $m$.