For questions about geometric shapes, congruences, similarities, transformations, as well as the properties of classes of figures, points, lines, angles.

learn more… | top users | synonyms

1
vote
2answers
11 views

How many unique vertices in octahedron based sphere approximation

Using a triangular facet approximation of a sphere based on Sphere Generation by Paul Bourke. We take an octahedron and bisect the edges of its facets to form 4 triangles from each triangle. ...
1
vote
1answer
45 views

Why is the following number always positive?

Consider two points in the Euclidean plane: $A=(A_1,A_2),B=(B_1,B_2)\in\mathbb{R}^2$, and some fixed real number $\lambda\in(0,1)$. The claim is that the following expression is always a positive ...
0
votes
2answers
30 views

Breaking down the equation of a plane

Could someone explain the individual parts of a plane equation? For example: $3x + y + z = 7$ When I see this I can't imagine what it's supposed to look like.
2
votes
2answers
19 views

2D geometric relation in a rectangle

I'm trying to implement the Sakoe & Chiba's global constraint for the Dynamic Time Warping algorithm but I'm stuck with a geometrical problem : I'm trying to find the value of d given a, b and c. ...
1
vote
1answer
38 views

Geometric proof of the Cross Product magnitude

Most proofs of the magnitude of the cross product are algebraic in nature, I find I learn best visually / geometrically. Is there a breakdown of the proof of the magnitude of the cross product using ...
0
votes
1answer
35 views

Need a formula / method to get a value between 0 and 1 if a point lies in an area between two rectangles

I'm trying to figure out a way to construct a formula or method for the following process: If Point E is inside the smaller rectangle (red area), the value should be 1. If Point E is outside the ...
0
votes
0answers
20 views

Find the amount of squares in the given picture [duplicate]

How many squares are in the above figure?
0
votes
2answers
31 views

Find the equation of the line perpenducular to the plane $x-y+z = 4$ [duplicate]

In geometry i am having difficulties in understanding how to find the equations.. e.g given $$x-y+z =4 $$which passes through the point $(2,1,5)$ when asked to find the equation of the line ...
4
votes
2answers
76 views

Prove that: $S_{XYZ}\geq \frac{1}{4}S_{ABC}$

$\triangle ABC$. Let $X\in BC; Y\in CA, Z\in AB$ such that $\angle YXZ= \angle BAC, \angle XZY=\angle ACB, \angle ZYX=\angle CBA$. Prove that: $S_{XYZ}\geq \frac{1}{4}S_{ABC}$ P/s: I have proved ...
6
votes
1answer
1k views

What is the name of this paradox?

What is the name of the mathematical paradox which is arises from the following? If we imagine a point on a two-dimensional coordinate system (line graph), which moves from the positive part of the ...
7
votes
4answers
439 views

Check if a point is inside a rectangle (not knowing the coordinates, but knowing distances to vertices)

I have to solve the following problem: I have 4 points (A, B, C, D) which form a rectangle, but I do not know their coordinates. I have another point (X), I do not know its coordinates either, but I ...
2
votes
4answers
102 views

I call them squares. They called them arrays. What do they mean?

So I was in C++, and we had third graders come today to play our programs. Whilst the others just drilled them with problems, my game was subtract a square. It was fun watching them discover that ...
2
votes
1answer
24 views

Interior Angle Embedded in a Triangle Embedded in a Circle

With only knowing the angles of $B$, $C$, and $D$ (shown above), is it possible to find the interior angle $A$? And if so, how?
-2
votes
0answers
40 views

Almost independent vectors- Where do they live on a manifold [on hold]

I am new to this thing. I am having the next question : Almost independent vectors- Where do they live on a manifold? In a manifold with larger dimmension? Tnks!So don't be tuff with me cause I am ...
1
vote
0answers
17 views

Hypersphere central angle

For a sphere, the relationship between steradian of a patch on the surface, and the central angle of the cone subtending that patch, is given by ...
1
vote
1answer
11 views

lattice : hypervolume and its associated hypersurface

Imagine I have a lattice with a point every one unit length in each direction. I'm interested in knowing how many points I have on the border (hypersurface) in any $D$ dimension. $1D$ for a length ...
0
votes
1answer
32 views

Where can I find a proof of this result?

Does anyone know where I can find a proof of the underlined statement? Newman states it without a proof, and I could see how he gets $\dfrac {\sigma}{n} + O\left(\dfrac{1}{n^{3/2}}\right)$. Any ...
1
vote
0answers
54 views

Abc is a triangle

Abc is a triangle (drawing of the triangle with measurements up the side of each side) Make a full size drawing of triangle abc in the space below The line AB has been drawn for you. Leave in all ...
0
votes
0answers
7 views

Mapping theorems in higher dimensions

The Poincaré mapping theorem states that given any two simply connected open domains of $\mathbb C$ that are neither empty nor equal to $\mathbb C$, there exists a unique conformal mapping from one to ...
-2
votes
2answers
36 views

looking for Radius /distance equation [on hold]

If I have a circle with the circumference of 40,000 km and I travel 100 km around, how far have I traveled along the radius? What is the equation for this? Many many thanks Phil
2
votes
1answer
31 views

n points can be equidistant from each other only in dimensions $\ge n-1$?

2 points are from equal distance to each other in dimensions 1,2,3,... 3 points can be equidistant from each other in 2,3,... dimensions 4 points can be equidistant from each other only in ...
2
votes
0answers
35 views

Is there an easier way to show that a kite has perpendicular diagonals using scalar products?

I want to use scalar products to prove that a kite has perpendicular diagonals. My attempt : Let $a,b,c,d$ vectors with $a+b+c+d=0$ and $a^2=d^2$ and $b^2=c^2$ Then, we get ...
1
vote
1answer
35 views

Prove that $x^2-y^2+xy-1=0$ is a ruled surface

I am studying for an analytic geometry, final but I am totally lost for this problem... We didn't even cover this topic in class (my prof didn't show up for class for two weeks) and I have no clue on ...
-1
votes
1answer
23 views

Two circular tangents

The total area of both circles is $230$ $m^2$, i need to find the radius of each circle. The circles are externally tangential and the distance from their centers are $11$$m$. Unable to upload ...
0
votes
1answer
24 views

Reducible cubic surface are always singular.

I want to prove that Any reducible cubic surface are always singular. A possible way may be to take a look at the intersection of the irreducible components. But I don't know how. Thanks for any ...
1
vote
0answers
15 views

How to visualize orientation of 3d objects

The way I visualize orientations of $1$- and $2$-dimensional objects is by an ant walking along a path. For a $1$d object (like a line/ line segment/ etc), just place the ant on the line and confine ...
0
votes
1answer
19 views

finding volume of an n-dimensional pyramid numerically

In my experiment I need to compute hypervolume/area from a set of points, let's start with a base case -- Triangle: In this case, I have 3 points in a 2D space and they make a triangle, $p_1 = ...
2
votes
1answer
46 views

Any curve of genus three is either hyperelliptic or trigonal?

A curve $C$ is said to be trigonal if it admits a rational function $f: C \to \mathbb{CP}^1$ of degree $3$. Is any nonsingular plane curve of degree four trigonal? Can the map be chosen so that it has ...
16
votes
1answer
96 views

Are there spaces that 'look the same' at every point, but are not homogeneous?

A metric space is homogeneous if for any two points there is a global isometry that maps one into the other. It is locally homogeneous if any two points have isometric neighborhoods, i.e. the space ...
1
vote
1answer
59 views

Find the equation of a cylinder

Find the equation of the cylinder that has directrix the curve: $x(t)=t, y(t)=t^2/2, z(t)=0$ and the generatrix is parallel to the line $${x-1\over 1}={y+2\over 1}={z\over 3}$$ I would really ...
0
votes
0answers
24 views

what would the equation of a torus be by making the circunference $(y-2)^2+ z^2 = 0$ and $x=0$ turn along the $z$ axis

What I understand of the question is that I have to, somehow, give the equation of the torus that results of spinning the circumference $$(y-2)^2 + z^2 = 0$$ and $$x=0$$ which as far as I know is just ...
0
votes
1answer
18 views

Clustering of vectors via inner product relationship

This might be an odd question, but suppose I have a lot of vectors $a_i\in\mathbb{R}^{3}$ (not necessarily unit) and for some unit vector $u\in\mathbb{R}^{3}$ I find $$ \sum_{i=1}^m ...
0
votes
1answer
45 views

What is wrong with my solution to this problem?

The base $ABCD$ of the figure has area $9$. The point $M$ divides the segment $AB$ on ratio $2$ and the edge $BF$ of length $2$ forms an angle of $60º$. Calculate $[CM,CB,BF]$, knowing that ...
1
vote
1answer
27 views

Lines on a quadric surface

I want to show that: Let $Q$ be a quadric surface of rank 3 in $\mathbf{P}^3$(over $\mathbf{C}$), then any line in $Q$ must pass through the only singular point of $Q$. I know that up to a transform ...
2
votes
1answer
17 views

Decomposition of hyper-rectangles into congruent simplices

Let $(a_1, \ldots, a_d) \in \mathbb{N}_+^d$ be positive integers and the semi-axes of the $d$-dimensional $\ell_1$-ellipse $$ E_{\bf a} := \{{\bf x} \in \mathbb{R}_{\geq 0}^d: \sum_{j=1}^d ...
3
votes
2answers
24 views

Find the number of possible points $R$.

$P(3,1),Q(6,5)$ and $R(x,y)$ are three points such that the angle $\angle PRQ=90^{\circ}$ and the area of the triangle $\triangle PRQ=7$.The number of such points $R$ that are possible is . $a.)\ ...
1
vote
3answers
54 views

A point D in a triangle ABC such that $\angle DAB= \angle DBC= \angle DCA$

I got this question from a student of mine, who is participating in a math olympiad competition: How can we construct a point D in a triangle ABC such that $\angle DAB= \angle DBC= \angle DCA$? I've ...
1
vote
1answer
9 views

How to find point on sphere from pitch and heading

I have a sphere of radius R and I would like to draw some vector positions on it given pitch and heading. I have a heading between 0 and 360 (0 being +x direction), and a pitch between -90 and 90 (90 ...
2
votes
2answers
46 views

Find h in terms of r

A sphere and a cylinder have equal volumes. The sphere has a radius 3r. The cylinder has radius 2r and height h. Find h in terms of r. I'm only 15, someone walk me through this as simple as ...
1
vote
3answers
33 views

How to find the radius if two circles intersect in two distinct points?

Question- if two circles $(x-1)^2+(y-3)^2=r^2$ and $x^2+y^2-8x+2y+8=0$ intersect in two distinct points , then find the range in which r exists I have these two circles $(x-1)^2+(y-3)^2=r^2$ and ...
2
votes
4answers
42 views

Geometric progression, how to find $x$

I tried many ways to solve this problem, but I can't! Please, someone explain how to solve this problem: If the sequence: $8, x, 50$ is a geometric progression, then $x = ?$
0
votes
0answers
5 views

Question regarding Calibration while using Phase Measuring Profilometry (PMP)

We are using PMP to create the 3d model of a real world object in a summer project. However, to actually use PMP we need to relate the camera and the projector parameters and coordinates. To ...
-1
votes
1answer
20 views

Standard integral Kähler form on $\mathbf{CP}^1 \times \mathbf{CP}^2$

$\newcommand{\Proj}{\mathbf{CP}}$What is the "standard integral Kähler form" on $\Proj^1 \times \Proj^2$? Does that mean Fubini-Study form on $\Proj^1$ and $\Proj^2$?
6
votes
4answers
291 views

Coordinates of the center of the circle

I am stuck on this problem: If the lines $y=x+\sqrt{2}$ and $y=x-2\sqrt{2}$ are two tangents of a circle and $(0,\sqrt{2})$ lies on this circle then what is the equation of the circle? I ...
1
vote
0answers
16 views

Analytic structures on $S^1$|

I am currently studying Haefliger's paper "Homotopy and Integrablity". During the last chapter, he applies his theory of $\Gamma$-structures to analytic codimension $1$ foliations. Throughout the ...
1
vote
2answers
18 views

How to find the area of the triangle formed by the lines $y=ax$ , $x+y-a=0$ and the $y$ axis?

I found out the intersection points $A( \frac{a}{1+a}$ , $\frac{a^2}{1+a} )$ and $B$ as $(0,a)$. Now, I don't know what to do next. Please explain in easy steps. Thank you!
2
votes
0answers
37 views

Tetrahedron and balls in space

A right tetrahedron and a ball arbitrarily located in space are given. It is allowed to reflect the tetrahedron from each of its faces. It is possible to place the center of the tetrahedron inside the ...
3
votes
4answers
75 views

Why do disks on planes grow more quickly with radius than disks on spheres?

In the book, Mr. Tompkins in Wonderland, there is written something like this: On a sphere the area within a given radius grows more slowly with the radius than on a plane. Could you explain ...
1
vote
3answers
31 views

Use Vectors To Show Three Vertices Belong to a Right Triangle

The Full Question Theorems Used This is what I call theorem 1: My Work This problem has two major steps as far as I can see. First, I must show that these are points of a triangle(not ...
2
votes
2answers
31 views

Find the equation of line in new co-ordinate system.

A line is represented by equation $4x+5y=6$ in the co-ordinate system with the origin $(0,0)$.You are required to find the equation of the straight line perpendicular to this line that passes ...