Olympiad problems

1993 Austrian-Polish Competition, 2

Consider all tetrahedra $ABCD$ in which the sum of the areas of the faces $ABD, ACD, BCD$ does not exceed $1$. Among such tetrahedra, find those with the maximum volume.

2017 China Team Selection Test, 1

Tags: 3d geometry , octahedron , solid geometry , geometry

Find out the maximum value of the numbers of edges of a solid regular octahedron that we can see from a point out of the regular octahedron.(We define we can see an edge $AB$ of the regular octahedron from point $P$ outside if and only if the intersection of non degenerate triangle $PAB$ and the solid regular octahedron is exactly edge $AB$.

2017 Yasinsky Geometry Olympiad, 5

Tags: 3-dimensional geometry , 3d geometry , solid geometry , rectangle , geometry

$ABCD$ is a rectangle. The segment $MA$ is perpendicular to plane $ABC$ . $MB= 15$ , $MC=24$ , $MD=20$. Find the length of $MA$ .

KoMaL A Problems 2024/2025, A. 894

Tags: sphere , solid geometry , insphere , 3d geometry , geometry

In convex polyhedron $ABCDE$ line segment $DE$ intersects the plane of triangle $ABC$ inside the triangle. Rotate the point $D$ outward into the plane of triangle $ABC$ around the lines $AB$, $BC$, $CA$; let the resulting points be $D_1$, $D_2$, and $D_3$. Similarly, rotate the point $E$ outward into the plane of triangle $ABC$ around the lines $AB$, $BC$, $CA$; let the resulting points be $E_1$, $E_2$, and $E_3$. Show that if the polyhedron has an inscribed sphere, then the circumcircles of $D_1D_2D_3$ and $E_1E_2E_3$ are concentric. [i]Proposed by: Géza Kós, Budapest[/i]

1998 Tuymaada Olympiad, 8

Tags: solid geometry , 3d geometry , right angle , geometry , pyramid

Given the pyramid $ABCD$. Let $O$ be the midpoint of edge $AC$. Given that $DO$ is the height of the pyramid, $AB=BC=2DO$ and the angle $ABC$ is right. Cut this pyramid into $8$ equal and similar to it pyramids.

2011 Sharygin Geometry Olympiad, 3

Tags: solid geometry , perpendicular , tetrahedron , geometry

Given two tetrahedrons $A_1A_2A_3A_4$ and $B_1B_2B_3B_4$. Consider six pairs of edges $A_iA_j$ and $B_kB_l$, where ($i, j, k, l$) is a transposition of numbers ($1, 2, 3, 4$) (for example $A_1A_2$ and $B_3B_4$). It is known that for all but one such pairs the edges are perpendicular. Prove that the edges in the remaining pair also are perpendicular.

1992 Austrian-Polish Competition, 7

Tags: right angle , 3d geometry , solid geometry , geometric inequality , geometry

Consider triangles $ABC$ in space. (a) What condition must the angles $\angle A, \angle B , \angle C$ of $\triangle ABC$ fulfill in order that there is a point $P$ in space such that $\angle APB, \angle BPC, \angle CPA$ are right angles? (b) Let $d$ be the longest of the edges $PA,PB,PC$ and let $h$ be the longest altitude of $\triangle ABC$. Show that $\frac{1}{3}\sqrt6 h \le d \le h$.

1997 Austrian-Polish Competition, 9

Tags: solid geometry , 3d geometry , volume , geometry

Given a parallelepiped $P$, let $V_P$ be its volume, $S_P$ the area of its surface and $L_P$ the sum of the lengths of its edges. For a real number $t \ge 0$, let $P_t$ be the solid consisting of all points $X$ whose distance from some point of $P$ is at most $t$. Prove that the volume of the solid $P_t$ is given by the formula $V(P_t) =V_P + S_Pt + \frac{\pi}{4} L_P t^2 + \frac{4\pi}{3} t^3$.

2021 Romania National Olympiad, 1

Tags: solid geometry , geometry

In the cuboid $ABCDA'B'C'D'$ with $AB=a$, $AD=b$ and $AA'=c$ such that $a>b>c>0$, the points $E$ and $F$ are the orthogonal projections of $A$ on the lines $A'D$ and $A'B$, respectively, and the points $M$ and $N$ are the orthogonal projections of $C$ on the lines $C'D$ and $C'B$, respectively. Let $DF\cap BE=\{G\}$ and $DN\cap BM=\{P\}$. [list=a] [*] Show that $(A'AG)\parallel (C'CP)$ and determine the distance between these two planes; [*] Show that $GP\parallel (ABC)$ and determine the distance between the line $GP$ and the plane $(ABC)$. [/list] [i]Petre Simion, Nicolae Victor Ioan[/i]

2017 China Team Selection Test, 1

Tags: solid geometry , 3d geometry , octahedron , geometry

Find out the maximum value of the numbers of edges of a solid regular octahedron that we can see from a point out of the regular octahedron.(We define we can see an edge $AB$ of the regular octahedron from point $P$ outside if and only if the intersection of non degenerate triangle $PAB$ and the solid regular octahedron is exactly edge $AB$.

2011 Sharygin Geometry Olympiad, 25

Tags: geometry , solid geometry , 3d geometry , tetrahedron

Three equal regular tetrahedrons have the common center. Is it possible that all faces of the polyhedron that forms their intersection are equal?

1996 Tuymaada Olympiad, 8

Tags: 3d geometry , solid geometry , tetrahedron , geometry , sphere

Given a tetrahedron $ABCD$, in which $AB=CD= 13 , AC=BD=14$ and $AD=BC=15$. Show that the centers of the inscribed sphere and sphere around it coincide, and find the radii of these spheres.

1998 Tuymaada Olympiad, 4

Tags: plane , dissecting plane , 3d geometry , solid geometry , geometry , tetrahedron

Given the tetrahedron $ABCD$, whose opposite edges are equal, that is, $AB=CD, AC=BD$ and $BC=AD$. Prove that exist exactly $6$ planes intersecting the triangular angles of the tetrahedron and dividing the total surface and volume of this tetrahedron in half.

2019 Oral Moscow Geometry Olympiad, 6

Tags: geometry , 3d geometry , solid geometry , angle

The sum of the cosines of the flat angles of the trihedral angle is $-1$. Find the sum of its dihedral angles.

1987 Mexico National Olympiad, 8

Tags: geometry , 3-dimensional geometry , solid geometry , perpendicular

(a) Three lines $l,m,n$ in space pass through point $S$. A plane perpendicular to $m$ intersects $l,m,n $ at $A,B,C$ respectively. Suppose that $\angle ASB = \angle BSC = 45^o$ and $\angle ABC = 90^o$. Compute $\angle ASC$. (b) Furthermore, if a plane perpendicular to $l$ intersects $l,m,n$ at $P,Q,R$ respectively and $SP = 1$, find the sides of triangle $PQR$.

2007 Sharygin Geometry Olympiad, 21

Tags: geometry , 3d geometry , solid geometry , area

There are two pipes on the plane (the pipes are circular cylinders of equal size, $4$ m around). Two of them are parallel and, being tangent one to another in the common generatrix, form a tunnel over the plane. The third pipe is perpendicular to two others and cuts out a chamber in the tunnel. Determine the area of the surface of this chamber.

1996 Czech and Slovak Match, 3

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

The base of a regular quadrilateral pyramid $\pi$ is a square with side length $2a$ and its lateral edge has length a$\sqrt{17}$. Let $M$ be a point inside the pyramid. Consider the five pyramids which are similar to $\pi$ , whose top vertex is at $M$ and whose bases lie in the planes of the faces of $\pi$ . Show that the sum of the surface areas of these five pyramids is greater or equal to one fifth the surface of $\pi$ , and find for which $M$ equality holds.

1972 Vietnam National Olympiad, 4

Tags: geometry , angle , tetrahedron , 3-dimensional geometry , solid geometry , 3d geometry

Let $ABCD$ be a regular tetrahedron with side $a$. Take $E,E'$ on the edge $AB, F, F'$ on the edge $AC$ and $G,G'$ on the edge AD so that $AE =a/6,AE' = 5a/6,AF= a/4,AF'= 3a/4,AG = a/3,AG'= 2a/3$. Compute the volume of $EFGE'F'G'$ in term of $a$ and find the angles between the lines $AB,AC,AD$ and the plane $EFG$.

1982 Brazil National Olympiad, 6

Tags: geometry , sphere , 3d geometry , solid geometry

Five spheres of radius $r$ are inside a right circular cone. Four of the spheres lie on the base of the cone. Each touches two of the others and the sloping sides of the cone. The fifth sphere touches each of the other four and also the sloping sides of the cone. Find the volume of the cone.

2012 Romania National Olympiad, 3

Tags: geometry , solid geometry , perpendicular

Let $ACD$ and $BCD$ be acute-angled triangles located in different planes. Let $G$ and $H$ be the centroid and the orthocenter respectively of the $BCD$ triangle; Similarly let $G'$ and $H'$ be the centroid and the orthocenter of the $ACD$ triangle. Knowing that $HH'$ is perpendicular to the plane $(ACD)$, show that $GG' $ is perpendicular to the plane $(BCD)$.

2013 IFYM, Sozopol, 2

Tags: solid geometry

Find the perimeter of the base of a regular triangular pyramid with volume 99 and apothem 6.

1982 Austrian-Polish Competition, 8

Tags: geometry , geometric inequality , 3d geometry , solid geometry , tetrahedron

Let $P$ be a point inside a regular tetrahedron ABCD with edge length $1$. Show that $$d(P,AB)+d(P,AC)+d(P,AD)+d(P,BC)+d(P,BD)+d(P,CD) \ge \frac{3}{2} \sqrt2$$ , with equality only when $P$ is the centroid of $ABCD$. Here $d(P,XY)$ denotes the distance from point $P$ to line $XY$.