hey i am new to this but here it goes.
proving that a parrallelogram is form. okay all the triangles are equillateral they have sides a,b,c,and d. all four sides are different. therefore, the triangles facing inward will never overlap themselves, thus the vertex points will exsist on those two trianlges.
also, the area of a trianlge = 1/2ab so this means that the trianlges vertex will always be 1/2 the lenght of the base line which is one of the sides of the quadralateral. now if you 90degree lines perpendicularlly to the quadrilarteral line the vertex with all of them will line up and also intersect in the middle of the quadrilateralWITH RESPECT THE THE ANGLES OF THE QUADRILATERAL LINES.
ther you have it in a very brief explanation.
by the way what clas is this for?