geometry assuming the parallel postulate of Euclid: in a plane, given a line and a point not on that line, there is exactly one line parallel to the given line through the given point.

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$\frac{4}{3}(1,-1,0)^T+\textrm{vect} \left((-1,1,-1)^T\right)$

Here I have a passage on the conclusion, but I do not understand it. the conclusion say : $f$ is the oriented axis of screw.. how I can from $\frac{4}{3} \begin{pmatrix}1\\1\\0 ...
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1answer
75 views

What's a good text to read before Coxeter's Geometry Revisited?

I am interested in reading Coxeter's famous text Geometry Revisited. It's not clear to me what the prerequisites for this text are, however. I'm sure I have enough mathematical maturity: I know ...
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1answer
614 views

Intersection of Two Circles

I have two circles as: $C_1: (x-x_1)^2+(y-y_1)^2=r_1^2$ and $C_2: (x-x_2)^2+(y-y_2)^2 =r_2^2$ and these circles have non-empty intersection. In other words $\sqrt{(x_1-x_2)^2+(y_1-y_2)^2}\leq ...
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1answer
3k views

About Euclid's Elements and modern video games

Update (6/19/2014) $\;$ Just wanted to say that this idea that I posted more than a year ago, has now become reality at: http://euclidthegame.com/ 12.292 users have played it in 96 different ...
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1answer
41 views

Maximum value of the area of triangle [on hold]

If two of the medians of a triangle have lengths x and y, what is the maximum value for the area?
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4answers
1k views

What is a point?

In geometry, what is a point? I have seen Euclid's definition and definitions in some text books. Nowhere have I found a complete notion. And then I made a definition out from everything that I know ...
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0answers
64 views

Average Degree of a Random Geometric Graph

A set of $N$ points are distributed randomly on a unit square with uniform distribution. Two points $\mathbf{p}_i$ and $\mathbf{p}_j$ are said to be connected if $\|\mathbf{p}_i - \mathbf{p}_j\| \leq ...
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1answer
21 views

Getting the intersection of a line and a plain

My line (2,1,10) goes through the plain with the normal (-2,3,8). Now I would like to calculate the intersection with following ...
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2answers
15 views

Why $(h,k)$ in equation $y= a(x-h)^2 +k$ is the vertex of a parabola?

As in the title , I know how to convert normal explicit equation to a vertex form equation by completing the square . But what is the reasoning behind why $(h,k)$ must be the vertex , but not other ...
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0answers
21 views

midpoint of the diagonal of the quadrilateral and rhombus

$EBA,FCB,GDC,HAD$ is a similar triangle which is drawn externally of quadrilateral $ABCD$, where the sides of quadrilateral $ABCD$ become the base of the similar triangle. Let $M,N,P,Q$ are midpoints ...
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3answers
361 views

Euclidean geometry exercise

I would like some help to solve this: Consider a triangle $\triangle ABC$ with $\angle A$ a right angle and $BC=20$. Divide $BC$ into four congruent segments, that is, take the points ...
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7answers
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Why Do The Axioms of Euclidean Geometry Not Need To Include the Definition of Space?

EDIT: update, I found that Euclid's axioms are not considered rigorous. David Hilbert did a full axiomatization of Euclidean Geometry (1899 in his book Grundlagen der Geometrie--tr. The ...
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1answer
15 views

Orthogonal Coordinates

I'm hoping someone could give me a good definition of "orthogonal coordinates." Attempts to find one online has left me only with a vague idea. A reference text would be appreciated.
5
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1answer
736 views

How to plot N points on the surface of a D-dimensional sphere roughly equidistant apart?

Let's say I have a D-dimensional sphere with a radius R. I want to plot N number of points evenly distributed (equidistant apart from each other) on the surface of the sphere. It doesn't matter where ...
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2answers
47 views

How to smooth a list of angles.

I'm not a math guy so maybe there is a super simple thing that my eyes cannot see. And sorry if my math terminology is not good at all. Please address me the right math terminology to use because ...
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1answer
14 views

Cyclic quadrilateral problem

In convex quadrilateral $ABCD$, $AB=2$, $AD=4$, and $2BC+CD=10$. If angle $DAC$ equals angle $DBC$, and the diagonals of $ABCD$ are perpindicular to each other, what is the area of $ABCD$? I have a ...
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24 views

Given $4$ points in the space, how do you check if an arbitrary point is within the area marked by those points?

Given $4$ arbitrary points in the space $A(x_1,y_1), B(x_2,y_2), C(x_3,y_3,), D(x_4,y_4)$, how do you check if an arbitrary point $X(x_5,y_5)$, is within the quadratic area marked by the $4$ points ...
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0answers
37 views

Archimedean Arbelos: seven circles with many properties [closed]

On a line $l$ there exists $3$ points $A,B,C$ where $B$ is located between $A$ and $C$. Let $ \Gamma1, \Gamma2 ,\Gamma3 $ be circles with $AC,AB,BC$ as diameter respectively. $BD$ is a segment, ...
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0answers
143 views

Why exactly is Bourbaki difficult? [closed]

I keep hearing people say that Bourbaki is difficult for most undergraduates but I still don't understand why. Surely if it starts from definitions/axioms then practically anyone should be able to ...
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1answer
71 views

Pentagon Forms a 10-sided Polygon Ratio Problem

Let $A_1A_2A_3A_4A_5$ be a regular pentagon with side length $1$. The sides of the pentagon are extended to form the $10$-sided polygon shown in bold in the picture that I have attached. Find the ...
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2answers
38 views

$ABCD$, $P$ is any interior point, $PA=24, PB=32, PC=28, PD=45$

could anyone tell me how to solve it? I have a convex quadrilateral $ABCD$, $P$ is any interior point, $PA=24, PB=32, PC=28, PD=45$ cm, I need to know the perimeter of $ABCD$. Thanks for helping. ...
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0answers
36 views

Triangle $ABC$ and equilateral triangles $ABC'$, $BCA'$ and $ACB'$.

We consider a triangle $ABC$ whose angles are less then $120°$ and construct the equilateral triangles $ABC'$, $BCA'$ and $ACB'$, exterior to $ABC$. $I$ denotes the intersection of $(AA')$ and ...
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3answers
46 views

Given an equilateral triangle, show that $MA + MC = MB$.

I have to solve the following problem: Consider an equilateral triangle $ABC$ and $\mathcal{C}$ its circumscribed circle. Let $M$ be a point located on the arc of the circle defined by $[AC]$ which ...
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19 views

Study the caracteristics of the transformation $f=r\circ t \circ h$.

Let $OABC$ be a square with $(\vec{OA},\vec{OC})=\frac{\pi}{2}$. Let $r$ be the rotation of center $B$ and angle $\alpha=\frac{\pi}{2}$, $t$ the translation of vector $\vec{CA}$, $h$ the homothetic ...
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1answer
23 views

Similarity of triangles?

The question is: "$ABCD$ is a quadrilateral in which angle $B =$ angle $C$ and $AC$ bisects angle $BAD$. If $BA$ and $CD$, when extended, meet at $E$, prove that $AD/DC = AE/BE$." I'm finding this ...
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1answer
17 views

Determining direction from three points on a line

I have a small geometry problem that for some reason I just can't get a grasp on. You're given three points on a line in 3D space, p1, p2, p3. (assume for simplicity that they're named ...
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0answers
12 views

Curves with a property about intersecting hyperplanes

I would like examples of curves in $\mathbb{R}^n$ with the property that any hyperplane of n-1 dimension through the origin intersects the curve at $\leq$ n points. In $\mathbb{R}$, the circle with ...
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1answer
24 views

How many points $P$ such that $\angle APB=\angle BPC=\angle CPA $ are there?

Given that $\triangle ABC$ is arbitrary. How many points $P$ such that $\angle APB=\angle BPC=\angle CPA $ are there?
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1answer
42 views

Why ternary diagrams work

I am trying to understand why ternary diagrams work. In order that the altitude criterion be valid, if I correctly understand, given equilateral triangle $ABC$, whose vertices I name as the three ...
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0answers
34 views

Necessary and sufficient condition for Euclidean geometry to hold? [on hold]

If a space obeys the axioms set out by Euclid in 'The elements' is this a necessary and sufficient condition for the geometry in that space to be Euclidean?
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1answer
42 views

“Polysticks” in 3d

Consider a finite set of three-dimensional Euclidean vectors with integer components. How many three-dimensional closed loops can I construct with them? How many of them are elementary, i.e., cannot ...
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1answer
46 views

Volume of the intersection of two tetrahedra

First, I am far from a mathematician, and this question may be easy, if that's the case, please don't hesitate to let me know. Suppose I have 2 tetrahedra (2 3D simplex), with known ...
4
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1answer
143 views

euclidean geometry books…

I consider myself poor in plane euclidean geometry. so I need a good geometry book which contains very good theory, and a collection a large number of solved problems, and the end of each part.This ...
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27 views

Is there any algorithm for finding the minimum distance to the complement of a convex set?

There have been some algorithms for finding the projection from a given point onto a convex set. This problem seems to be quit easy because of the convexity of the set. However, in the case of finding ...
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1answer
93 views

Is there a name for the recursive incenter of the contact triangle?

Recently, I became aware that there are many more triangle centers than the four I learned about in school. This reminded me of a thought I had when I first learned about the incenter: what point ...
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0answers
10 views

Dual Objects and Symmetries

In the study of symmetries of platonic solids (tetrahedron, cube, octahedron, ..), I came across the following. Group of rotational symmetries of a cube is $S_4$. Since octahedron is dual to cube, ...
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21 views

Mathematics-Oriented 4-D Glossary?

Is there somewhere a comprehensive glossary of words or phrases describing geometric concepts or objects in the Euclidean (not Einsteinian) fourth dimension? I have seen a glossary which purported to ...
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1answer
30 views

Euclidean problem of geometry

Let the two quadrilaterals ABCD and EFGH been given: Let's take these hypothesis: $AD = EH$ $A\hat{B}D=A\hat{C}D=E\hat{F}H=E\hat{G}H$ $AC=EG$ The triangle ABD is isosceles and equal to the ...
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0answers
6 views

Intersection/union of hyperballs in Minkowski space

I'm trying to manipulate hyperballs bounded by the Minkowski distance. What I would like to do is take the intersection/union of two hyperballs and then find the smallest hyperball which covers the ...
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1answer
40 views

calculate points coordinates on plane from their distances matrix

Given a list of points on a plane is simple to generate a distances list between each pair of points. Pseudo Code: ...
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0answers
35 views

Cyclic convex quadrilateral property

Let $ABCD$ be a cyclic convex quadrilateral, and let $P$ be the intersection of the diagonals. Show that $\frac{PB}{PD}=\frac{AB}{AD}\cdot \frac{CB}{CD}$. I guess I need to use Ptolemy's ...
9
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3answers
202 views

Scaling and rotating a square so that it is inscribed in the original square

I have a square with a side length of 100 cm. I then want to rotate a square clockwise by ten degrees so that it is scaled and contained inside the existing square. The image below is what I'm ...
0
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1answer
31 views

What is the fundamental theorem on discrete groups of Euclidean spaces?

I have been reading the book Using Algebraic Geometry by David A. Cox, John Little, Donal O'Shea for a university project. I am not clear as to what exactly in meant by the phrase "the fundamental ...
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1answer
368 views

Find locus of points relating to an ellipse

I would like to find the equation of the following locus. For a big circle C centered at (0,0), the locus of points that the sum of distances to Y-axis and to C is 1, say in the first quadrant, is ...
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3answers
53 views

Every reflection is an isometry proof

The theorem is that every reflection $R_{S}$ in an affine subspace $S$ of $\mathbb{E}^{n}$ is an isometry: $R_S:\ \mathbb{E}^{n} \rightarrow \mathbb{E}^{n}:\ x \mapsto R_{S}(x) = x + 2 ...
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1answer
75 views

At most $2n$ vectors, the angle between which $\geq\pi/2$.

In a previous question it is proved that in $\mathbb R^n$ there are at most $n+1$ vectors, the angle between which $>π/2$. How to prove that there are at most $2n$ vectors, the angle between which ...
2
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1answer
44 views

Alternative word for Euclidean Geometry

If Euclid has only collected the geometry stuffs while books of the other geometer have been burnt, calling the main branch of geometry under name of him might look academically unethical for some ...
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1answer
25 views

If three circles have two common points, prove that every circle that is orthogonal to two circles is also orthogonal to third.

Three circles are given $k_1$,$k_2$,$k_3$ that have two common points A and B. Prove that every circle $k$ that is orthogonal to circles $k_1$,$k_2$, is also orthogonal to $k_3$. Here is my proof ...
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0answers
11 views

Contructions of a rectangular hyperbolar and a inscribed parabola [on hold]

Please see: http://mathworld.wolfram.com/ChaslessPolarTriangleTheorem.html http://forumgeom.fau.edu/FG2004volume4/FG200427.pdf I proposed problem construction of a rectangular hyperbolar and a ...
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20 views

ABC is a triangle, D is a point in the triangle. E is the midpoint of BD. AB=BC, angle ABD= angle DBC=35 degrees, angle ACD=25 degrees. Angle BAE=?

I tried to solve this problem but couldn't. I just know that here, angle BDC= 100 degrees, angle BAC= 40 degrees, AB^2+AD^2=2(AE^2+BE^2) and AB/AD={sin(angle DAE)}/{sin(angle BAE)}