Parallelograms I

Look at a garden gate, a folding clothes-drying rack, or the criss-cross scissor-lift on a lorry — they all hold their shape because of parallelograms. In your O/L paper, geometry riders that ask you to prove something is a parallelogram, or to find a missing angle or side, come up every single year and carry good marks. The beauty of this topic is that from one small definition, a whole family of useful facts follows — and once you see how they connect, the proofs almost write themselves.