Нова формула за лице на четириъгълник (четиво за VII клас)

Abstract

Here we derive formula for the area of a quadrilateral with angle 45º between the diagonals, which is a special case of a more general formula for the area of a quadrilateral (Haimov 2022). Along the way we obtain formulas for the area of triangles with angle of 45º/135º. We also give formulas for the area of specific kinds of pentagons and hexagons, in the form of problems.