Questions tagged [geometry]

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

Geometry is one of the classical disciplines of math. It is derived from two Latin words, "geo" + "metron" meaning earth & measurement. Thus it is concerned with the properties and relations of points, lines, surfaces, solids, and higher dimensional analogs. Since its earliest days, geometry has served as a practical guide for measuring lengths, areas, and volumes, and geometry is still used for this purpose today. Geometry is important because the world is made up of different shapes and spaces.

Geometry has applications to many fields, including art, architecture, physics, as well as to other branches of mathematics.

Sub-fields of Contemporary Geometry:

$1.\quad$ Algebraic Geometry – is a branch of geometry studying zeroes of multivariate polynomials. It includes the linear and polynomial algebraic equations used for finding these sets of zeros. The applications of algebraic geometry include cryptography, string theory, etc.

$2.\quad$ Discrete Geometry – is concerned with the relative positions of simple geometric objects, such as points, lines, triangles, circles etc.

$3.\quad$ Differential Geometry – uses techniques of algebra and calculus for problem-solving. The applications of differential geometry include general relativity in physics, etc.

$4.\quad$ Euclidean Geometry – The study of plane and solid figures on the basis of axioms and theorems including points, lines, planes, angles, congruence, similarity, solid figures. It has a wide range of applications in computer science, modern mathematics problem solving, crystallography etc.

$5.\quad$ Convex Geometry – includes convex shapes in Euclidean space using techniques of real analysis. It has application in optimization and functional analysis in number theory.

$6.\quad$ Topology – is concerned with properties of space under continuous mapping. Its application includes consideration of compactness, completeness, continuity, filters, function spaces, grills, clusters and bunches, hyperspace topologies, initial and final structures, metric spaces, metrization, nets, proximal continuity, proximity spaces, separation axioms, and uniform spaces.

$7.\quad$ Plane Geometry – This wing of geometry deals with flat shapes which can be drawn on a piece of paper. These include lines, circles & triangles of two dimensions.

$8.\quad$ Solid Geometry – It deals with $3$-dimensional objects like cubes, prisms, cylinders & spheres.

Reference:

https://en.wikipedia.org/wiki/Geometry

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equilateral triangle; $3(a^4 + b^4 + c^4 + d^4) = (a^2 + b^2 + c^2 + d^2)^2.$

In equilateral triangle ABC of side length d, if P is an internal point with PA = a, PB = b, and PC = c, the following pleasingly symmetrical relationship holds: $3(a^4 + b^4 + c^4 + d^4) = (a^2 + b^2 + c^2 + d^2)^2.$ Please prove this…
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Reconciling 'intersecting planes' and 'linear transformation' interpretations of matrices

I've learned in linear algebra class that an $n \times m$ augmented matrix can be thought of as a collection of n planes in $\mathbb {R}^m$ . If the matrix is invertible, the planes all intersect at a single point. If it has infinite solutions, two…
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Prove that the distance between a black and a white dot is one

I just read this article about some tough interview questions. One of the questions (allegedly given in an interview for a Technology Analyst position in Goldman Sachs) was: There are infinite black and white dots on a plane. Prove that the …
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Can I prove Pythagoras' Theorem using that $\sin^2(\theta)+\cos^2(\theta)=1$?

In any right-angled triangle, the area of the square whose side is the hypotenuse (the side opposite the right angle) is equal to the sum of the areas of the squares whose sides are the two legs (the two sides that meet at a right angle). The…
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What is the shape of the convex $n$-gon which gives the maximum of a function?

Supposing that the length of every edge of the convex $n$-gon $P_1P_2$$\cdots$$P_n$ is 1, what is the shape of the $n$-gon which gives the maximum of the following function $A_n$? $$A_n=\sum_{1\le{i}\lt{j}\le{n}}|P_iP_j|^2$$ Here, $|P_iP_j|$ is the…
mathlove
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A Geometric Construction problem that's got me stumped since 1975

Hello, I'm a retired civil engineer from Greece. Ever since I was a student I have really liked maths. I had to solve this problem during my first year at NTUA, back in 1975! I couldn't solve it then and my question remains: what is the solution?…
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Similar Right Triangles on Sides of Triangle and Treating BC as a Pascal Line to Obtain a Cyclic Hexagon

Proposition. Let $ABC$ be an acute triangle. Exterior points $B’$ and $C’$ are such that $\triangle AB’B$ and $\triangle AC’C$ are similar with right angles at $B’$ and $C’$, respectively. If $M$ is the midpoint of $BC$ and $C’M \cap AB = D$, then…
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Number of Regions in the Plane defined by $n$ Zig-Zag Lines

Fellows of Math.SE, I have been scratching my head at a solution to an exercise in Donald Knuth's Concrete Math. Here is the problem: Here is the solution (I hid it in case someone wants to solve this on their own) Given $n$ straight lines that…
A.E
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Show that U,V and H are colinear

We are given a regular icosagon as below: I wanna prove that the red line exists. I know that $U$ is the incenter of $\triangle TLB$ ($T,U,G$ are collinear) I know that $V$ is the incenter of $\triangle GMC$ ($M,V,E$ are collinear) I tried to use…
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Hopf fibration and homotopy of spheres

Let $$ S^3 \to S^7 \to S^4 $$ an the Hopf fibration. We con consider the induced sequence in homotopy $$ \pi_i(S^3) \to \pi_i(S^7) \to \pi_i(S^4) \to \pi_{i-1}(S^3) \to \pi_{i-1}(S^7) \to \cdots $$ So we have, using the suspension homomorphism that…
ArthurStuart
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Intersection of parabola and circle

Is it possible to place circle and a parabola on the plane so that their intersection consist of exactly two points, one point being a point of tangency, and the other point a transversal intersection?
ekrez
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How do you derive that the inradius in a right triangle is $r=\frac{a+b-c}2$?

If we have a right triangle then the inradius is equal to $$r=\frac{a+b-c}2,$$ where $c$ is the hypothenuse and $a$ and $b$ are the legs. This formula is mentioned in various places and it can be useful both in geometric problems and in problems on…
Martin Sleziak
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Proving "If there exists a line containing exactly $n$ points, then any line contains exactly $n$ points" from basic axioms of incidence

I have to show that the following theorem can be proven using the axioms cited below: If there exists a line that contains exactly $n$ points, then any line contains exactly n points, and any point has exactly $n$ lines that contain it. The axioms…
DougL
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How these circles are congruent?

Here is a problem involving curvilinear incircles and mixtilinear incircles. Let a triangle$\triangle$$ABC$ have circumcircle $\gamma$.It's A-Excircle tangency point at side$BC$ is $D$ Let $\gamma_1$ be the circle tangent to $AD$,$BD$,$\gamma$…
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Can every tree with total length $2$ be covered by a semi-disc of radius $1$?

Can every tree with total length $2$ be covered by a semi-disc of radius $1$? If the tree is actually a curve, or the convex hull of the tree is a triangle, I know this is correct after some attempts. But for the general case, I have no idea.
Minghui Ouyang
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