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

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2answers
55 views

The bisector of $\angle BAC$ of triangle $\Delta ABC$ cuts $BC$ at $D$

The bisector of $\angle BAC$ oF triangle $\Delta ABC$ cuts $BC$ at $D$ and circumcircle of triangle at $E$. if $$AD=5 \text{ cm} ,\ DE=3 \text{ cm},\ AC=4 \text{ cm}, $$ then what is the length of ...
4
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0answers
56 views

A movement requires 1 dimension, a rotation requires 2 dimensions, a what requires three dimensions?

Movement A zero-dimensional object cannot move. A one-dimensional object can move in one dimension (the x-axis). A two-dimensional object can move in two dimensions (the x-axis and y-axis). A three ...
3
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1answer
45 views

$D, E, F$ are respectively projection of $O$ on $BC, CA, AB$. Prove that $\cot{\angle ADB} + \cot{\angle BEC} + \cot{\angle CFA} =0$

Let $O$ be an arbitrary point located inside the triangle $ABC$. Let $D, E, F$ be (respectively) the projections of $O$ on $BC, CA, AB$. Prove that $$\cot{\angle ADB} + \cot{\angle BEC} + ...
9
votes
3answers
268 views

What does it mean to be $0.9-$Dimension?

We can visualize $1\mathrm D$, $2\mathrm D$, $3\mathrm D$ and we can think of a higher $M-\mathrm {Dimension}$ where $M\in \mathbb N^+$as a vector. But I recently learned that there are non integer ...
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0answers
44 views

Create solid torus with geometric algebra

I have created an algorithm for tracking a vessel's centerline in 3 dimensions, and traveling through that centerline. My supervisor asked me whether I could add a small theoretical section on ...
2
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2answers
49 views

O.I.M. polygon inequality

I am trying to prove an inequality which was used to prepare the Romanian O.I.M. team. I seem to lack ideas on how to tackle this problem. We take a convex polygon $P_1\ldots P_{n+2}$ and consider ...
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0answers
9 views

How to derive the relation $T=S_1$ for the equation of a chord of an ellipse whose midpoint is given?

How to derive the relation $T=S_1$ for the equation of a chord of an ellipse whose midpoint is given ?
1
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1answer
29 views

Projection of the XY plane.

Is there a way to project the infinite Complex plane to either the Poincare disk or the unit disk - for all values of x + iy ?
0
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1answer
34 views

A formula to calculate the partial volume of a capsule or tank?

We are trying to ascertain the correct formula discussed in this post. The volume formula for a capsule (a cylinder with a hemisphere at both ends) is, $$V_c = \pi r^2 H + \frac{4}{3}\pi r^3\tag1$$ ...
2
votes
4answers
82 views

Find the area of a triangle given the radius of its incircle and a tangential point

A friend gave me recently the following interesting problem and I would like to share a couple of solutions. Any additional contributions are welcome. A triangle $\vartriangle ABC$ is given and we ...
2
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1answer
43 views

One special case of Helly's theorem (for $\text{radius}=1$ circles)

There are $n$ points on the plane. Any $3$ of them can be covered with a radius $1$ circle. Prove that there is a radius $1$ circle that covers all the points. Came to this when tried to prove an easy ...
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1answer
32 views

Find the value of $x$ below

$AB=DC$, Find the value of $x$ I tried with Law of Sines but I get different answer every time
7
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1answer
85 views

Largest rectangle bounded under a function

Let $f$ be a positive monotonically increasing real function in $[0,1]$. Let $F$ be the area under the curve of $f$ ($F=\int_0^1{f(x)dx}$) For every $x\in[0,1]$, let $G(x)=f(x)*(1-x)$ = the area of a ...
1
vote
1answer
93 views

Find the value of $h$ from a Kepler-type equation

$$V = \frac{0.5r^{2}\cdot \cos^{-1}(\frac{r-h}{r})\cdot 2-\sin\big(\cos^{-1}(\frac{r-h}{r})\cdot 2\big)}{10^{6}}\tag1$$ This is the equation to find the volume of liquid in a tank in the shape of a ...
7
votes
1answer
38 views

Product to vertices in triangle maximal

Suppose we're given a triangle $ABC$. At which interior point $T$ is the product of distances $|AT|\cdot |BT|\cdot |CT|$ maximal? Is it a known point, like the centroid or incenter?
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0answers
29 views

Geometry problem… [on hold]

GH=OP Find $x$ [1 any help guys ??
0
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0answers
22 views

Generalization of Lines and Planes

Let $a_1,a_2,\dots,a_n$ be constants that are not all zero. An equation defines a line if and only if it can be written: $a_1x_1+a_2x_2+a_3=0$ An equation defines a plane if and only if it can be ...
0
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2answers
16 views

If the area of a trapezoid is 63 square feet, find the height. [on hold]

The bases of a trapezoid are one and three feet longer than the height respectively. If the area is 63 square feet, find the height.
1
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2answers
421 views

The base of a triangular prism ABC.A'B'C' is an equilateral triangle with lengths a,

The base of a triangular prism ABC.A'B'C' is an equilateral triangle with lengths a, and the lengths of its adjacent sides also equal a. Let I be the midpoint of AB and $B'I \perp (ABC)$. Find the ...
0
votes
1answer
30 views

What condition do I have set to have $x_j$ for $j=1,…,n$ to be non-negative?

I have $\sum_{j=1}^n q_{jj}(x_j-y_j)^2\le1$ What condition do I have set to have $x_j$ for $j=1,...,n$ to be non-negative? The book I am reading says $\sqrt{q_{jj}}\ge1/y_j$ but why? edit: ...
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2answers
30 views

$\frac{y-b}{r}=\frac{y}{s}$ to $y$ for finding the closest point on a line, from a point.

$$r=sy^2-sby$$ How do I get $y$ on one side? Originally I had: $\dfrac{y-b}{r}=\dfrac{y}{s}$
1
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2answers
18 views

Create Ellipse From Eccentricity And Semi-Minor Axis

So I am given the eccentricity of an ellipse and the radius semi-minor axis as well as the center of the ellipse. So in the example below we know the center of the ellipse is at ( 0, 0 ) and the ...
0
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0answers
38 views

Segments of a hypotenuse

The hypotenuse of a right triangle is divided into 2 segments by the altitude to the hypotenuse The sum of the greater segments on the hypotenuse of 2 disimilar right triangles is equal to the ...
1
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2answers
878 views

Calculate Triangle Ground using Height and Top Angle

Is it possible to calculate the ground of a triangle only using the height and top angle. Click here to see a poorly draw sketch of what I'm trying to calculate. So is it possible and how, to ...
1
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1answer
33 views

Arbitary plane curve.

Does an arbitrary curve in the plane necessarily pass through a rational point? That is, a point of the form $(a,b)$ where $a$ and $b$ are rational numbers.
20
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11answers
9k views

3D software like GeoGebra

Does it exist a free interactive geometry software, like GeoGebra, which works for 3D geometry? I would be able to draw spheres, great circles, and so on.
3
votes
2answers
20 views

Does the radius of the quadrant pass from the center of the inscribed circle?

In the following picture: The smaller circle is inscribed inside the quadrant, whose radius (OB) is 8. The original question (but not the question of this post) is that "find the radius of the ...
1
vote
3answers
53 views

Regular n sided polygon

$A_1A_2A_3....A_{18}$ is a regular 18 sided polygon.B is an external point such that $A_1A_2B$ is an equilateral triangle.If $A_{18}A_1$ and $A_1B$ are adjacent sides of a regular n sided polygon.Then ...
2
votes
1answer
97 views

compute the bisecting normal hyperplane between two $n$-dimensional points.

I have two points $\mathbf{x_1}$ and $\mathbf{x_2}$, where $\mathbf{x_i}=\{x^i_1, x^i_2, \ldots, x^i_n\}$. I need to find a normal hyperplane $P$ that goes through the midpoint of $\mathbf{x_1}$ and ...
2
votes
2answers
49 views

Total area for a natural nested set of convex polygons.

Suppose we have a convex polygon $P_0$ with $n$ given vertices, and we want to "nest" polygons $P_j$ for $j > 0$ by taking the midpoints between edges of $P_{j-1}$ as the vertices. For a regular ...
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0answers
18 views

How to find truncated cylinder\ungula Volume

How would I calculate the volume at a height in an upside down truncated cylinder? Everything I find online shows a truncated cylinder being flat and level on its base, but what if the cylinder is ...
1
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0answers
38 views

How do I calculate the area the Wigner Seitz cells cover in a square?

It's my first time here, so I appologise in advance if I break any rules through this post. So I have a Cartesian Lattice spanning across the Euclidean plane and a unit square. The lattice points ...
1
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1answer
14 views

Countable intersection of Cut Locuses is always empty?

If $C_p(M)$ is the cut locus of some $p\in M$ in some Riemannian Manifold $M$, then does there always exist a countable collection of points $\{p_n\}_{m \in \mathbb{N}}$ such that: \begin{equation} ...
1
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0answers
27 views

Intuitive way to find the minimum surface in $\mathbb R^3$?

Suppose we have two arbitrary closed curves which intersect neither each other nor themselves. By intuition, I guess that the minimum surface ending at boundaries is unique and it is achieved by this ...
3
votes
3answers
61 views

Does the centroid of a triangle ever fall outside of its Morley's triangle?

Let $T$ be a triangle, and $M$ its (first) Morley triangle:                     (Image from Bruce Shawyer web page.) Q1. Does the ...
0
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1answer
27 views

inner product of positive semi definite symmetric matrices [on hold]

I have a positive semi definite symmetric matrix $X$, $(n\times n)$. let $X=vv^T$ s.t $\|v\|=1$. I came to a point where I am stuck to show which is: $v^TYv=\langle X,Y\rangle$ (How to show this ...
3
votes
1answer
297 views

Prove that there is only one way to make a square using all six tangram pieces

I am pretty sure there is only one way to make a square from the six tangram pieces: How can I prove this is the only way respecting all symmetries?
0
votes
2answers
30 views

Width of rotated plane

I'm trying to get the width of a rotated plane, but my knowledge of trig functions didn't really help me get what I want. I have a plane, that is $310$ units wide, and is $200$ units away from the ...
1
vote
1answer
764 views

Find the line in $\mathbb{R}^3$ that passes through the point $(1,2,-3)$ and is parallel to the vector $u=(4,-5,1)$.

Find a vector equation and parametric equation of the line in $\mathbb{R}^3$ that passes through the point $(1,2,-3)$ and is parallel to the vector $u=(4,-5,1)$. Find two points on the line that are ...
2
votes
1answer
35 views

Circle bisecting the circumference of another circle

If the circle $x^2+y^2+4x+22y+l=0$ bisects the circumference of the circle $x^2+y^2-2x+8y-m=0$,then $l+m$ is equal to (A)$\ 60$ (B)$\ 50$ (C)$\ 46$ (D)$\ 40$ I don't know the condition when one ...
3
votes
1answer
35 views

Prove a rotating window shade won't break my window when raised to a specific height.

I have a large lampshade that covers my window to block out sunlight. It has a metal rod sewn in at the bottom to weigh it down, but it's aluminum, so it can rock in wind. We recently had a flash ...
1
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1answer
364 views

Internal polygon formed by drawing diagonals in a regular polygon

In an n-sided (n>4) regular polygon, label the vertices {0, 1, ..., n-1}. For each vertex i, draw a pair of diagonals: from i to (i+2) mod n and from i to (i-2) ...
1
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1answer
425 views

Calculating centre of rotation given point coordinates at different positions

I am trying to work out if the centre of rotation of a measured sphere is actually at 0,0 or slightly offset from the centre. The situation is as follows: I have a machine tool with a table that ...
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2answers
39 views

Finding matrix representation of an Ellipsoid [on hold]

I have a $2$-dimensional ellipsoid centered at $(1,2)$. The axes are parallel to $y=x$ and $y=-x$, and it passes through points $(-1,0)$, $(3,4)$,$(0,3)$,$(2,1)$. I would like to find the symmetric ...
3
votes
1answer
27 views

Intersection of Random Subspace and Hypercube

Suppose that $A \subset \mathbb{R}^n$ is a random linear subspace of dimension $k < n$. I am interested in the event that $A$ intersects the hypercube $[-1,\ 1]^n$ at a specific face. In other ...
1
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1answer
96 views

Calculus Apostol Exercise 1.11 number 6

I just don't know how to start with this problem, it seems obvious but I'm just stumped on how to start the proof. Any suggestions?
0
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2answers
40 views

Can you do this to find circumference from area of a circle

If you divide the circumference by $2$, does it equal the area divided by the radius? That is, do you have $C/2 = A/r$ for any circle? ...
1
vote
2answers
15 views

Geometrical calculation to enlarge the height of rotated rectangle

There is a polygon (rotated rectangle) that defined by 4 corner points in 2D coordinate system. Does anyone help me with the fast (minimum trigonometry operations) algorithm to change its height by ...
0
votes
1answer
426 views

How to convert a right angled triangle into a equilateral triangle?

I want to use the Apophysis program to make a right angled sierpinski triangle into an equilateral triangle. But how can i do so? i have tried the second picture one but that is not correct.
8
votes
2answers
454 views

A star shaped open set in $\mathbb{R}^n$ is diffeomorphic to $\mathbb{R}^n$

After googling this fact (which is used in Bott and Tu to show the existence of good covers of manifolds) I've gotten the impression this is somewhat difficult to prove. But I also came across this ...