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Browse College Euclidean Geometry
Stars indicate particularly interesting answers or
good places to begin browsing.
 Angles as Turns [05/29/2003]

How can angles be negative?
 Angles of Reflection [03/13/2001]

I am trying to prove that the incoming angle of a ray from one focus
within a ellipse is equal to the angle of the outgoing ray to the other
focus...
 Area, Circumference of an Ellipse [7/29/1996]

How do I calculate the area and circumference of a given ellipse?
 Are Angles Dimensionless? [08/31/2003]

If you look at the dimensions in the equation arc length = r*theta, it
appears that angles must be dimensionless. But this can't be right.
Or can it?
 Area of a Crescent [06/18/2001]

When observing a total eclipse of the sun we need to determine the area
of the sun that has not been covered by the moon.
 Area of an Ellipse Cut by a Chord [05/26/2000]

How can you calculate area of the part of an ellipse cut off by a chord,
if you know the major and minor axes, and the chord?
 Area of an Ellipsoid [09/28/2001]

How do you calculate (or even estimate) the area of an ellipsoid that is
neither oblate nor prolate?
 Area of a Reuleaux Triangle [06/13/2002]

Could you please help me find a formula to find the area of a
Reuleaux triangle?
 Ball Bouncing off a Line Segment [05/04/2001]

If you take an arbitrary line on a 2D plane, e.g. x1y1  x2y2, then take
a point that moves about the plane, say pxpy, can you tell if this point
has crossed the line at any time?
 Barycentric Calculus [01/06/1999]

How does barycentric calculus compare with trilinear or cartesian
calculus?
 Bestfitting Line to a Number of Points [04/25/2001]

I have a number of points on a plane and want to find a line that best
fits through the points, minimizing the sum of squares of the distances
of each point from the line.
 Bretschneider's Theorem and Cyclic Quadrilaterals [11/30/2000]

Can you prove Bretschneider's Theorem for the area of a quadrilateral?
Also, can you show that any quadrilateral with supplementary opposing
angles can be inscribed in a circle?
 Catenary Curve [03/30/1999]

Find the vertex of a catenary curve.
 Centroid  Center of Gravity [03/25/2002]

Can a triangle have a unique centre of gravity?
 Circle Slicing Cylinder: 3 Dimensions, 3 Approaches [08/10/2016]

A C++ programmer seeks ways to find the points where a cylinder and circle might
intersect in 3space. With parameterizations aplenty, Doctor George suggests three
possible methods.
 Classifying Shape Based on Coordinate Points [01/03/2000]

How can I design an algorithm to classify shapes based on a relatively
small set of (x,y) coordinates that describe the boundary of a closed
object?
 Connected Sets in Topology [04/22/1998]

Exploring connected sets with examples in Euclidean space.
 Constructing the Trisection of an Angle [07/20/2008]

Why can you construct the trisection of some angles and not of others?
 Coordinate Geometry  Goat in a Circular Field [8/22/1996]

A goat is tethered by a rope of length L to a point on the circumference
of a circular field of radius R. Find L in terms of R if the goat can
graze exactly half the area of the field.
 Curve Fitting Algorithm [6/20/1996]

Can you explain "least squares approximation"?
 Definitions of Edge and Face in 2D and 3D [10/10/2008]

What is the 'official' definition of 'edge'? Specifically, is an edge
restricted to the intersection of two noncoplanar faces or do two
dimensional shapes have edges? I'm also curious about a definition of
'face'. How many faces does a twodimensional shape have?
 Deriving a 2D Rotation Matrix [11/17/2009]

How can I geometrically derive this rotation matrix?
x' = cos(theta) sin(theta) x
y' sin(theta) cos(theta) y
 Deriving Parametric Equations for Cissoid of Diocles [02/12/2005]

How do you derive the parametric equations for the Cissoid of Diocles?
 Desargues' Theorem [07/03/1998]

Why is Desargues' two triangle theorem easy to prove in three dimensions
but impossible in two dimensions?
 Determining Length of Material Remaining on a Roll [11/24/2003]

Is there a mathematical formula to determine the length of material on
a roll, given the outside diameter of the core, the outside diameter
of the whole roll, and the thickness of the material (determined by a
micrometer)?
 Distance between Points of Tangency [02/16/2003]

Two circles, one of radius 5, the other of radius 8, intersect at
exactly one point, and the center of each circle lies outside the
other circle. A line is externally tangent to both circles. Find the
distance between the two points of tangency.
 Drawing an Ellipse Using Conjugate Diameters [12/07/2005]

Can you explain how to use Conjugate Diameters to draw an ellipse?
 Euclid's Fifth Postulate [6/24/1996]

I am interested in finding some theorems, axioms, or postulates similar
to Euclid's Fifth Postulate.
 Finding Intersections of Two Ellipses [06/17/2005]

As part of a computer program, I'm working on the problem of detecting
the intersection of two ellipses given the length of their axes and
coordinates of their centers on the XY plane. The ellipses can be
oriented so that their major axis is either vertical or horizontal.
Is there a way to do this that does not require computationally
expensive techniques such as brute force checking of points?
 Finding Reflection Points within a Rectangle [03/24/2005]

Given a beginning point and an ending point inside a rectangle, I'm
trying find a formula, algorithum or calculation that would tell me
where on the rectangle I would have to aim a laser from the given
beginning point so that it reflects off exactly two sides of the
rectangle and connects with the given ending point.
 Finding the Center of a Circle [12/26/1996]

Given a circle of radius R with center point unknown, a line with
equation Y = mx+b and a line at Y = .08, find the x,y coordinates of the
points of tangency where the two lines intersect the circle.
 Find the Perimeter of the Rectangle [09/06/2002]

Two circles of radii 9 and 17 centimetres are enclosed within a
rectangle with one side of length 50 cm. The two circles touch each
other, and each touches two adjacent sides of the rectangle. Find the
perimeter of the rectangle.
 Fitting an Arc to a Point and Two Tangent Lines [11/14/2004]

I'm wondering how to fit an arc to a point and two circles. There is a
vertical line segment of length 0.3 m, with a circle of radius 0.037 m
coming from the midpoint of that line, and another circle of radius
0.104 m coming from the bottom of the segment. I want to fit a 3
point arc to the top of the segment and the tangents of the two circles.
 Folium of Descartes and Parametric Equations [11/23/1998]

How do you plot an implicit function, such as the folium of Descartes,
with the equation y^3 + x^3 = 3xy?
 Formula for Radial Latitudes and Longitudes [10/15/2002]

I want to make a radius search tool that returns all places based on
latitude/longitude within a certain radius of a given place.
 General Equation for Intersections of Line and Ellipse [03/13/2007]

I am trying to find a general equation for the intersection points (if
there are any) of a line and ellipse. The line is defined by two
points (x0,y0) and (x1,y1) and the ellipse is defined as x^2/a^2 +
y^2/b^2 = r^2. I tried writing an equation of the line and then
substituting it into the ellipse equation, but it got too complicated.
 The Goat In the Field Problem [05/24/1997]

A farmer tethers a goat to the circumference of a circular field. What
ratio of field radius to length of rope must he use so that the goat can
graze only half the area?
 Goat Tied by a 10Meter Rope [11/28/2001]

A goat is tied to the corner of a 5by4meter shed by a 10meter rope.
What area is grazed by the goat? If the shed is a circle with radius r,
and the rope is 2r, what is the area grazed?
 Golden Spiral [03/23/1998]

What is the equation of the Golden Spiral?
 Hyperbolic Geometry [03/24/2003]

Explain this assumption: Assuming that Euclidean geometry is
consistent, had any of the failed attempts to prove Euclid's 5th
Postulate from the other axioms succeeded, they would have actually
completely destroyed Euclidean geometry as a consistent body of
thought.
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