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

50021 questions
4
votes
5 answers

A question about an equilateral triangle

Suppose that $\triangle ABC$ is an equilateral triangle. Let $D$ be a point inside the triangle so that $\overline{DA}=13$, $\overline{DB}=12$, and $\overline{DC}=5$. Find the length of $\overline{AB}$.
4
votes
0 answers

Mascheroni Construction of the Heptadecagon

Since Gauss proved that the heptadecagon is constructible with ruler and compass, there were found plenty of ways of constructing it, some of them are pretty clever (e.g. DeTemple and Ma Long). On the other hand, there is a theorem due to Mascheroni…
André Porto
  • 1,855
4
votes
1 answer

Show that $CD\parallel AB$ with square $FEBD$

As the figure shows, $FEBD$ is a squre. $AE=GE$, $FA=FB$ and $CD=BA$. Show that $CD\parallel AB$. I have found that $\triangle BGD$ must be an equilateral triangle, but I have no proof yet. Please help. It's better to offer a synthetical solution…
mengdie1982
  • 13,840
  • 1
  • 14
  • 39
4
votes
3 answers

Drawing all chords between six points on a circle, prove that only one triangle is formed in the circle's interior.

Motivating problem: https://artofproblemsolving.com/wiki/index.php/2010_AMC_10A_Problems/Problem_22 If we draw a triangle in the interior of a circle, it is straightforward to show that the triangle can be constructed by the intersection of chords…
user670718
4
votes
1 answer

Prove that points O, P, Q are collinear

Let $\triangle ABC$ be an acute triangle, with $O$ as its circumcenter. $H$ is the foot of the perpendicular from $A$ to the line $BC$, and points $P$ and $Q$ are the feet of the perpendiculars from $H$ to the lines $AB$ and $AC$,…
4
votes
3 answers

How can I get precisely a certain cubic cm by changing the following factors?

By a calculation of the size of the cubic $\text {cm}$ which its sizes are: $3.8330 \times 3.8330 \times 5.17455$, I got a volume of $76.02 cm^3$. How can I get precisely "$76.04$" $\text {cm}^3$, by changing the first two mentioned factors (i.e.…
4
votes
1 answer

Smallest box to fit cuboid in.

You have a cuboid with dimensions of $30$ by $8$ by $4$ inside a box (cube). Every point of the cuboid touches some side of the box. What is the smallest box you can fit this cuboid in? I need the size of the box's edge
4
votes
1 answer

Could an irregular triangular-base pyramid be constructed with 4 identical irregular triangles? Why or why not?

Suppose we have a regular triangular-base pyramid(A.K.A.: a Tetrahedron). Obviously every single triangular side on a tetrahedron "meets flush" with every single other side. Or, another way of putting it, the object can be constructed without holes…
4
votes
1 answer

$ax+by=20$,$ax+by=30 \quad ab = ?$

Let $a,b$ be real positive numbers such that $b>a$.If the area of the region that between the two lines $ax+by=20$,$ax+by=30$ and the positive part of the axes $X,Y$ is equal to $10$ unit square.How to find $a\cdot b$
4
votes
2 answers

Is the notion of circle necessary for proving some problem of angle?

As shown in the image for a plane geometric problem: Could we prove $\angle ACD=\angle ABD$ without using the notion of circle? It could seem easy if we have the notion of circle. But if we have no the notion of circle?
user49413
  • 115
4
votes
1 answer

Is there a way to put 5 points on the surface of the sphere so that they are indistinguishable?

Is it possible to put 5 points on the surface of the sphere such that, for every pair of them, say A and B, there is an isometric transformation of the space such that A is mapped onto B, B is mapped onto A, and the convex cover of all 5 points is…
4
votes
4 answers

Showing locus of points is a hyperbola

I've having trouble understanding what the question is trying to ask. And I am not sure how to start to answer the question. The diagram below shows what happens for waves on the surface of a pond. If you drop a stone in the point at the point…
4
votes
2 answers

About the Three Reflections Theorem

I recently solved this exercise from Hartshorne's Classical Geometry. Given three lines $a$, $b$, $c$ through a point $O$, show that there exists a unique fourth line $d$ such that $$\sigma_c\sigma_b\sigma_a=\sigma_d,$$ where $\sigma$ denotes the…
yunone
  • 22,333
4
votes
3 answers

What are positive and negative curvature when working in non-Euclidean geometry?

Can you give me the sense I can find the difference between positive and negative curvature in non-Euclidean geometry better? I am asked today by my friend and I want to give her a good practical explanation. Thank you very much!
4
votes
2 answers

Diameter of Three Inscribed Circles

What is the diameter of a circle in which are inscribed three smaller identical circles, two of which are on one side of a chord, the third on the other side? This problem came up when cutting a log into billets for turning table legs. I tried…
Roger
  • 51