Parallelograms II

In Lesson 15 you learned what a parallelogram is and the neat properties that follow from it. This lesson turns that knowledge around. Now the question is: someone hands you a four-sided shape — how do you prove it is a parallelogram? A carpenter checking a cabinet door, a surveyor mapping a land plot in your village, an engineer designing a folding gate — none of them can measure everything. They measure the two or three things that are easy, and from that they must be sure. This lesson gives you four separate tests, any one of which settles the matter.