assuming yer nomenklachuer av RSTU go around rim av parallelogran in order
(klok wise or kounter klokwise), a line fro R tu T is the diagonal.
(Also hav other triangel tween 2 other points)
Eether diagonal split parallelogram intu 2 identikal triangels
Both hav 2 equal sides: 1 side is the base, other is the angeled side at the end
Other side is the diagonal yu just drawd...it is kommon side tu both triangels.
3 sides the same pruev 2 trianges identikal.