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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Can you make a sphere out of a plane?

I had this idea to build a model of Earth in Minecraft. In this game, everything is built on a 2D plane of infinite length and width. But, I wanted to make a world such that someone exploring it could think that they could possibly be walking on a…
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Modelling the "Moving Sofa"

I believe that many of you know about the moving sofa problem; if not you can find the description of the problem here. In this question I am going to rotate the L shaped hall instead of moving a sofa around the corner. By rotating the hall…
newzad
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A goat tied to a corner of a rectangle

A goat is tied to an external corner of a rectangular shed measuring 4 m by 6 m. If the goat’s rope is 8 m long, what is the total area, in square meters, in which the goat can graze? Well, it seems like the goat can turn a full circle of radius…
space
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Can a row of five equilateral triangles tile a big equilateral triangle?

Can rotations and translations of this shape perfectly tile some equilateral triangle? I've now also asked this question on mathoverflow. Notes: Obviously I'm ignoring the triangle of side $0$. Because the area of the triangle has to be a…
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Can manholes be made in other shapes than circles, that prevent the cover from being able to fall down its own hole?

Circular manholes are great because the cover can not fall down the hole. If the hole were square, the heavy metal cover could fall down the hole and kill some man working down there. Circular manhole: Can manholes be made in other shapes than…
hpekristiansen
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Is there a size of rectangle that retains its ratio when it's folded in half?

A hypothetical (and maybe practical) question has been nagging at me. If you had a piece of paper with dimensions 4 and 3 (4:3), folding it in half along the long side (once) would result in 2 inches and 3 inches (2:3), which wouldn't retain its…
Pyraminx
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Calculating the volume of a restaurant take-away box that is circular on the bottom and square on the top

Having a bit of a problem calculating the volume of a take-away box: I originally wanted to use integration to measure it by rotating around the x-axiz, but realised that when folded the top becomes a square, and the whole thing becomes rather…
Nemui
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Why is a circle in a plane surrounded by 6 other circles?

When you draw a circle in a plane you can perfectly surround it with 6 other circles of the same radius. This works for any radius. What's the significance of 6? Why not some other numbers? I'm looking for an answer deeper than "there are…
John Smith
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Why is the Möbius strip not orientable?

I am trying to understand the notion of an orientable manifold. Let M be a smooth n-manifold. We say that M is orientable if and only if there exists an atlas $A = \{(U_{\alpha}, \phi_{\alpha})\}$ such that $\textrm{det}(J(\phi_{\alpha} \circ…
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Why is a full turn of the circle 360°? Why not any other number?

I was just wondering why we have 90° degrees for a perpendicular angle. Why not 100° or any other number? What is the significance of 90° for the perpendicular or 360° for a circle? I didn't ever think about this during my school time. Can someone…
P K
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20 circles in the plane, all passing through the origin

Suppose I draw $20$ circles in the plane, all passing through the origin, but no two tangent at the origin. Also, except for the origin, no three circles pass through a common point. How many regions are created in the plane?
laser01
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Escaping infinitely many pursuers

The fugitive is at the origin. They move at a speed of 1. There's a guard at every integer coordinate except the origin. A guard's speed is 1/100. The fugitive and the guards move simultaneously and continuously. At any moment, the guards only move…
Eric
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What's a proof that the angles of a triangle add up to 180°?

Back in grade school, I had a solution involving "folding the triangle" into a rectangle half the area, and seeing that all the angles met at a point: However, now that I'm in university, I'm not convinced that this proof is the best one (although…
Joe Z.
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Volumes of n-balls: what is so special about n=5?

The volume of an $n$-dimensional ball of radius $1$ is given by the classical formula $$V_n=\frac{\pi^{n/2}}{\Gamma(n/2+1)}.$$ For small values of $n$, we have $$V_1=2\qquad$$ $$V_2\approx 3.14$$ $$V_3\approx 4.18$$ $$V_4\approx…
Andrey Rekalo
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What is the meaning of the third derivative of a function at a point

(Originally asked on MO by AJAY.) What is the geometric, physical, or other meaning of the third derivative of a function at a point? If you have interesting things to say about the meaning of the first and second derivatives, please do so.
Gil Kalai
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