The common 4-sided figures learnt in Primary 5 are parallelogram, rhombus, trapezium and isosceles trapezium . Master the properties/characteristics of these 4-sided figures! (1) Parallelogram AB parallel with DC. That's why we can say adjacent angles, angle a + angle d = 180 degree , similarly angle b + angle c = 180 degree . DA parallel with CB. So adjacent angles, angle d + angle c = 180 degre e, similarly angle a + angle b = 180 degree . In addition, angle b = angle d (opposite angles), angle c = angle a . (2) Rhombus AB parallel with DC and DA parallel with CB. That's why rhombus has all the same properties as parallelogram above. In rhombus, all sides are equal , hence angle d1 = angle d2 = half of the angle d . (3) Trapezium AB parallel with DC. Therefore, adjacent angles, angle a + angle d = 180 degree , similarly angle b + angle c = 180 degree . NOTE: (very important !) adjacent angle d + angle c is NOT EQUAL to 180 degree adjacent angle a +...