0
votes
0answers
9 views

To construct a right triangle given the hypotenuse and sum of two legs

NOTE: I want a hint only. A compass and a straightedge construction:Given a hypotenuse and the sum of lengths of the legs,we need to construct a right triangle. MY TRY: From any ray $BE$, ,let ...
0
votes
1answer
31 views

Draw a picture of a cube with edges a+b, and show it cut by planes that divide each edge into a segment of length a and a segment of length b.

I am reading through 4 pillars of geometry and I need some help with this question. Draw a picture of a cube with edges a+b, and show it cut by planes (parallel to its faces) that divide each edge ...
1
vote
2answers
34 views

Maximize the inradius given the base and the area of the triangle

BdMO 2013 Secondary: A triangle has base of length 8 and area 12. What is the radius of the largest circle that can be inscribed in this triangle? Let $A,r,s$ denote the area,inradius and ...
0
votes
3answers
68 views

Perpendicular lines inside and outside a circle

No trigonometry allowed. Let $\Delta ABC$ be inscribed inside a circle.Let $P$ be a point on the circle.Let $PD$ and $PE$ be perpendiculars on on $BC$ and $AC$ respectively.Let $DE$ when extended ...
0
votes
1answer
29 views

Heptagonal tesselations

Are there any tesselations of the Euclidean plane that use only regular polygons such that one of them is a heptagon? If so, what is the tesselation that uses the fewest different types of polygon ...
6
votes
4answers
687 views

which axiom(s) are behind the Pythagorean Theorem

There are many elementary proofs for the Pythagorean Theorem, but no matter they use areas, similarities, even algebraic proofs, it is not straightforward to tell why it is true tracing back to the ...
0
votes
2answers
37 views

Prove $\sin^2 A = \sin^2 B \sin^2 C - 2\sin B \sin C \cos A$

I am asking for help with this proof: Given $\triangle ABC$. Prove that $\sin^2 A = \sin^2 B+ \sin^2 C - 2\sin B \sin C \cos A$
1
vote
0answers
28 views

Geometric conditions equivalent to a set being the unit circle for some norm

Here's the question, as in the textbook (Real Mathematical Analysis, Pugh). The unit ball with respect to a norm $||\, \cdot \,||$ on $\mathbb{R}^2$ is $$ \{ v \in \mathbb{R}^2 : ||\, v \,|| ...
0
votes
2answers
27 views

Triangles with common centroid

Consider the points $A',B',C'$ on the sides $BC,CA,AB$ of a triangle $ABC$ respectively, such that $BA'/A'C=CB'/B'A=AC'/C'B$. Show that the triangles $ABC$ and $A'B'C'$ share a common centroid.
1
vote
0answers
32 views

Solution for the value of an angle of a triangle ABC

Find value of angle m< DBC Where $$BD=DC=AC$$ $$2(m\langle BAC)=14(m\langle ABD)=7(m\langle BCD)$$ I tried hard but im out of ideas now, I know the answer is 20 but I want to know how, thanks ...
-1
votes
1answer
18 views

Perimeter of the triangle

$BO$ and $CO$ are angle bisectors of triangle $ABC$. How much is perimeter of the triangle $AMN$?
0
votes
0answers
21 views

Partition of the 3d space with circles?

Does it exist a partition of the 3d space with circles of positive radium? I know the answer is no for a plane, but I can not transpose my demonstration to the space and I have no clue on how to do ...
0
votes
1answer
47 views

How long is the diagonal of this trapezoid?

Given a trapezoid $abcd$, with $|ab| = 1$, and angles $\angle dab = 3\theta/4$, $\angle abc = (\pi + \theta)/2$, $\angle bcd = (\pi - \theta)/2$, and $\angle cda = \pi - 3\theta/4$ (see figure below), ...
3
votes
2answers
47 views

Is a quadrilateral with one pair of opposite angles congruent and the other pair noncongruent necessarily a kite?

If convex quadrilateral ABCD has congruent angles A and C and the other pair of angles B and D are not congruent, is ABCD necessarily a kite (two pairs of consecutive congruent sides but opposite ...
0
votes
1answer
66 views

Are Euclidean distances a monotone function of inner products?

Does the sum of all pairs of inner-products of k vectors (real) have to decrease if the sums of Euclidean distances between all pairs of $k$ vectors happens to decrease? Similarly-if decrease is ...
1
vote
1answer
38 views

Tangents to a circle

For this construction, how would you show that the perimeter of the triangle $CDF$ is equal to $2BC$? Please include steps and whatnot.
1
vote
1answer
40 views

volume of the solid

Using geometry, calculate the volume of the solid under $z = \sqrt{49- x^2- y^2}$ and over the circular disk $x^2+ y^2\leq49$. I am really confused for finding the limits of integration. Any help?
10
votes
1answer
94 views

A problem with concyclic points on $\mathbb{R}^2$

I am thinking about the following problem: If a collection $\{P_1,P_2,\ldots,P_n\}$ of $n$ points are given on the $\mathbb{R^2}$ plane, has the property that for every $3$ points $P_i,P_j,P_k$ in ...
1
vote
1answer
240 views

Expected value of the distance between 2 uniformly distributed points on circle

I have the following problem (related to Bertrand): Given a circle of radius $a=1$. Choose 2 points randomly on the circle circumference. Then connect these points using a line with length $b$. ...
0
votes
0answers
28 views

Conic equation from cone/plane intersection

In an orthonormal cartesian frame $(O; \vec{x}, \vec{y}, \vec{z})$ consider: an infinite plane $P$ defined by: a point $p = (p_x, p_y, pz)$ an normal vector $\vec{n} = (n_x, n_y, n_z)$ a cone $C$ ...
0
votes
1answer
34 views

Geometry question considering triangles and cyclic quads

Let $\triangle ABC$ be a triangle and $P$ a point on the circumcircle of this triangle. Let $U$, $V$, and $W$ be the projections of $P$ onto the three sides of the triangle. Show that the points $U$, ...
1
vote
1answer
35 views

Geometrically prove that for a point on a diameter…

Geometrically prove that for a point on a diameter between the center point and the perimeter of a circle, the distance between this non-center point is the shortest distance to the perimeter. So $A$ ...
1
vote
0answers
28 views

Which kinds of geometry have an angle measure?

It is possible to construct alternative planes to the usual Euclidean plane $\mathbb{R}^2$ by replacing the real numbers $\mathbb{R}$ with other ordered fields $F$. Depending on the choice of $F$, ...
0
votes
1answer
34 views

Determine missing angle in polygon

I'm trying to figure out this question: Determine the measure of angle a I'm guessing $a=96\unicode{0186}$ using the following work: $$a = 180 - 84 = 96 $$ ...
0
votes
1answer
43 views

Linear distance is proportional to angular distance, why?

Im my Fourier series book, the following is stated: We may specify the position of a point on the circle by its angular coordinate $\theta$, measured from some fixed base point. Since linear distance ...
5
votes
2answers
181 views

Vertex-transitive polytope with large facet

Consider a vertex-transitive convex polytope with a facet containing more than the half of all vertices. Does it already have to be a simplex or are there other examples? I am particularly interested ...
-3
votes
1answer
29 views

what is the X coordinate of point with Θ-90 degree [closed]

what is the X coordinate of point with Θ-90 degree ? 1- 0 2- 1.414 3- 7.07 4- infinity if I used this x = r sin(Θ)cos(ϕ) how can start I mean that if I substitution with Θ in sin with Θ-90 ...
0
votes
1answer
17 views

what is the X component of the point , where the spherical coordinates of point are (100,30,60)?

The spherical coordinates of point are $(100,30,60)$, what is the X component of the point $30$ $43.3$ $50$ $75$ I know that in the spherical coordinates, $$x = r \sin(\theta) \cos(\phi),$$ so ...
0
votes
1answer
35 views

spherical coordinates

spherical coordinates of point are $(10,20,30)$, the distance between the point and the origin of coordinate system is ? 1- $10$ 2- $14.4$ 3- $20$ 4- $30$ I know that the distance between two ...
1
vote
1answer
23 views

Prove that orthogonality in Euclidean space is geometrically perpendicularity?

Is this simply true by definition (that is, taken as axioms?) How would one to prove that for $||\vec{x}||=1$ and $||\vec{y}||=1$, if $(\vec{x},\vec{y})=0$, then $\vec{x}\perp\vec{y}$? In other ...
0
votes
1answer
49 views

there concurrent lines, perpendicular to the sides of a triangle

Given a triangle $\Delta ABC$. Let $A_1,B_1,C_1$ be points on the circum circle of $\Delta ABC$ such that $AA_1\parallel BC , BB_1 \parallel AC, CC_1 \parallel AB$. Through the points $A_1,B_1,C_1$ ...
3
votes
1answer
96 views

I am looking for a proof of the “ begonia theorem”.

Let $D$, $E$, $F$ be points on respective (extended) sides $\overleftrightarrow{BC}$, $\overleftrightarrow{CA}$, $\overleftrightarrow{AB}$ of $\triangle ABC$, such that $\overleftrightarrow{AD}$, ...
2
votes
1answer
65 views

Area of triangle inside triangle

In triangle $ABC$ we choose 3 points $D,E,F$, such that $\overline{AD} = \frac 13 \overline{AB}, \overline{BE} = \frac 13 \overline{BC}, \overline{CF} = \frac 13 \overline{CA}$. Draw segments ...
1
vote
1answer
113 views

Maximizing the perimeter of a triangle inside a square

BdMO 2014: We have a square $ABCD$ of side length 5.We take a point $E$ on $AD$ and $F$ on $AB$ so that $\angle FCE=45^\circ$. What can be the maximum perimeter of $\triangle AEF$? I can ...
3
votes
0answers
62 views

How to determine the length of $x$ in this sketch?

Consider the following construction: Assume that you are given the lengths of the sides $a,b,c,d,e$ as well as the size of the angle $\phi$. The task is to determine the length of $x$ in terms of the ...
0
votes
1answer
120 views

How to do you find missing vertix in right triangle in a graph?

![enter preformatted text here][2]I'm graphing a line segment. The end points are $A$ and $B$. Then I"m using $10 \%$ of the length of line segment $AB$ to form ...
3
votes
3answers
105 views

How would you prove that the graph of a linear equation is a straight line, and vice versa, at a “high school” level? [duplicate]

This is something I've been wondering about. Namely, I've always accepted "on intuition" that the equation $$ax + by = c$$ is, when graphed, a line. You can plot the points $(x, y)$ satisfying the ...
2
votes
0answers
61 views

Shining a laser into a mirror maze

I tried to formulate the following problem in a more mechanical way involving soccer balls, but the physics got too unrealistic. I know that what follows could be made more precise, but I hope the ...
0
votes
1answer
35 views

Given a triangle $ABC$, with altitude $AD$ and circumcircle radius $R$, show that $AD = 2R\sin\ B\sin\ C$.

Given a triangle $ABC$, with altitude $AD$ and circumcircle radius $R$, show that $$AD = 2R\sin\ B\sin\ C.$$ I'm a bit stumped as to how the altitude of $ABC$ and the circumcircle radius interact ...
1
vote
2answers
61 views

Geometry problem about angles and triangles

I've been working on this problem for a while. It doesn't seem to hard, but I cannot reach a satisfying solution. The triangle $ABC$ is isosceles with base $\overline{AC}$. A point $O$ is also ...
2
votes
2answers
97 views

How do I prove that $CP > \frac 1 2 (AC+BC-AB)$? [closed]

Given is the triangle $ABC$ with point $P$ on side $AB$. How do I prove that $$CP > \frac 1 2 (AC+BC-AB)?$$
1
vote
1answer
72 views

A problem of forming equal angles in plane geometry

C and D are two points on the same side of a straight line AB. Find a point X on AB such that angles CXA and DXB are equal.
1
vote
2answers
215 views

Formula to find the third point of triangle when two points and all sides are known?

I am writing a program in java. I looking for formula to determine the 3rd point in a triangle if the length of all sides and the coordinates of two points are known.
1
vote
2answers
65 views

If $ABCD$ is a cyclic quadrilateral, then $AC\cdot(AB\cdot BC+CD\cdot DA)=BD\cdot (DA\cdot AB+BC\cdot CD)$

If $ABCD$ is a cyclic quadrilateral, then $$ AC\cdot(AB\cdot BC+CD\cdot DA)=BD\cdot (DA\cdot AB+BC\cdot CD) $$ I tried using many approaches, but I could not find a proper solution. Can anyone please ...
4
votes
0answers
124 views

Area of a equilateral triangle given distances of a point in the triangle from the vertices [closed]

A point $D$ inside an equilateral triangle $PQR$. $D$ is located at a distance of $3$cm, $4$cm and $5$cm respectively from $P$, $Q$ and $R$. What is the area of the triangle $PQR$?
6
votes
2answers
106 views

Given a triangle find the length of BC

Given an ABC triangle , if $AB+AD=4$, find the length of BC.
2
votes
3answers
130 views

Moscow Math Olympiad 1973

In every polyhedron there is at least one pair of faces with the same number of sides. Solution: Let $N$ be the greatest number of sides in a face of a given polyhedron. Then the number of ...
0
votes
0answers
28 views

Area of intersection between square and annulus

The annulus' larger radius is $1$, smaller radius is $r>0$, and center is $(0,0)$. The square's sides are parallel with the axes, the lower left corner's coordinates are $(a,b)$, and the upper ...
0
votes
0answers
57 views

Ratios in a rhombus

NOTE: I am NOT looking for a full answer,just a hint. Last problem on this question. BdMO 2013 Chittagong: Let $ABCD$ be a rhombus.Let $G$ be a point outside the rhombus such that GE is ...
4
votes
3answers
72 views

Proof that a line cuts in half the area of a parallelogram iff it goes through the intersection of the diagonals?

I read a theorem in a book which says that a line bisects a parallelogram iff it goes through the intersection of the diagonals. The edge case of this is of course if the line is one of the diagonal ...