Online Geometry: Isosceles Triangles, Theorems and Problems

Isosceles Triangle - Table of Content

Euclid's Elements Book I, 23 Definitions. One-page visual illustration.
Euclid's Elements Book. Index

Euclid's Elements Book I, Proposition 5: (Pons Asinorum) Isosceles triangles

Isosceles Triangle 80-20-80. Index.

Perpendicular Bisector.

Langley's Problem Adventitious angles. 20° isosceles triangle with animation.
 

Geometry Problem 694.
Triangle, Angles, 30 Degrees, Auxiliary Construction, Mind Map, Polya.

Geometry Problem 688.
Triangle, Angles, 10, 20, 30, 40, 60 Degrees, Measure, Mind Map, Polya.

Geometry Problem 685
Isosceles Triangle, 100-40-40 Degrees, Angles, Congruence.

Geometry Problem 628
Cyclic Quadrilateral, Congruence, Parallel Lines.

Geometry Problem 627
Cyclic Quadrilateral, Congruence, Concyclic Points.

Problem 577.
Isosceles triangle, Midpoint, Perpendicular, Concyclic points, Circle.

Proposed Problem 469.
Isosceles triangle, Midpoint, Transversal, Congruence.

Proposed Problem 439.
Isosceles triangle, Cevian, Incenter, Angles, Circle.

Proposed Problem 419.
Isosceles triangle, Cevian, Incircle, Excircle, Inradius, Exradius.

Proposed Problem 380.
Triangle, Excenter, Parallel to a side, Angle, Congruence,
Isosceles triangle.

Proposed Problem 379.
Triangle, Excenter, Parallel to a side, Angle, Congruence,
Isosceles triangle.

Proposed Problem 378.
Triangle, Incenter, Parallel to a side, Angle, Congruence,
Isosceles triangle.

Proposed Problem 372.
Circles, Common Internal and External Tangent, Angles.
Isosceles triangle.

Proposed Problem 358.
Isosceles triangle 80-80-20, Circle, Angles, Congruence.

Proposed Problem 274.
Isosceles Triangle, 80-80-20, Angles.

Proposed Problem 224. Viviani's theorem, Isosceles triangle, Altitude, Distances, Point on the extension of the base.

Proposed Problem 223. Viviani's theorem, Isosceles triangle, Altitude, Distances.

Bottema's Theorem.
Invariance of an isosceles right triangle.
Triangle and Squares with Interactive Geometry Software
Step-by-Step construction, Manipulation, and animation.

Kurschak's Tile and Theorem. Jozsef Kurschak (Hungary, 1864-1933) An elegant and a purely geometric way of finding the area of a regular dodecagon.
32 congruent isosceles triangles.

Proposed Problem 325.
Isosceles triangle, Altitude, Cevian, Incircle, Excircle.

Mascheroni Construction with compass alone. Midpoint of a segment
Based on similarity of isosceles triangles.

Proposed Problem 252.
Rectangle, isosceles right triangle, square, area.

Proposed Problem 60: Isosceles triangle

Geometry Online Glossary . Isosceles triangle
Geometry Glossary based on the new New York State mathematics standards initiative.

MindMap of Euclid's Elements Book I
Triangles by Side: Equilateral, Isosceles, Scalene. 

Humans Hard-wired into brain for Geometry.
The Munduruku subjects, tested by a native speaker of Munduruku working with a linguist, were asked to identify the image that was ‘weird’ or ‘ugly.’ For example, to test the concept of right angles, a slide shows five right triangles and one isosceles triangle. The isosceles triangle is the correct answer.

Problem 19. Isosceles Right triangle, perpendicular, and Excenter.

Proposed Problem 13. Triangle, cevian, equal segments, and angles.
Auxiliary construction: Isosceles triangle.

Proposed Problem 2. Triangle and angles..
Auxiliary construction: Isosceles triangle.