A triangle is a polygon. It is a closed 3-sided shape made up of straight lines. The ‘tri-’ part of the word triangle means three. A triangle has 3 sides and 3 angles. Interior angles are the ...

Triangles ABC and GHI are congruent. Triangle GHI is the same as ABC but has been rotated 90° anticlockwise. Triangle DEF is a different size. Image caption, Line segments AB and GH are the same ...