Formulas for areas of rectangles, triangles and parallelograms. KS3 Bitesize: Maths - Perimeter - Introduction. KS3 Bitesize: Maths - Area - Triangles. KS3 Bitesize: Maths - Area - Introduction. KS2 Bitesize Maths - 2D shapes, Page 6 : Read. Elaboration.