Angles in a triangle sum to 180° proof (video) | Khan Academy
In that video, Sal took the fact that angles in a triangle sum to 180 degrees because this was true and he was trying to prove that corresponding angles are equal.
Relationships in triangles and quadrilaterals | Khan Academy
Theorems concerning triangle properties Learn Properties of congruence and equality Angles in a triangle sum to 180° proof
Congruence | High school geometry | Math | Khan Academy
Learn what it means for two figures to be congruent, and how to determine whether two figures are congruent or not. Use this immensely important concept to prove various geometric …
Triangle exterior angle example (video) | Khan Academy
An exterior angle is formed outside a triangle by extending a side. Its measure is equal to the sum of the two non-adjacent interior angle measures, thanks to the fact that all angle measures in a …
Intro to angle bisector theorem (video) | Khan Academy
The Angle Bisector Theorem states that when an angle in a triangle is split into two equal angles, it divides the opposite side into two parts. The ratio of these parts will be the same as the ratio …
Angles of a triangle (review) | Geometry (article) | Khan Academy
Want to learn more about the interior angles in triangles proof? Check out this video.
Congruence | Geometry (all content) | Math | Khan Academy
Learn what it means for two figures to be congruent, and how to determine whether two figures are congruent or not. Use this immensely important concept to prove various geometric …
Similarity | High school geometry | Math | Khan Academy
Learn what it means for two figures to be similar, and how to determine whether two figures are similar or not. Use this concept to prove geometric theorems and solve some problems with …
Angle-angle triangle similarity criterion - Khan Academy
Use dilations and rigid transformations to show why a pair of triangles with at least two pairs of congruent corresponding angles must be similar.
Proofs concerning isosceles triangles (video) | Khan Academy
Sal proves that the base angles in isosceles triangles are congruent, and conversely, that triangles with congruent base angles are isosceles. He also proves that the perpendicular to …