[Not to be turned in]
 Use vectors and dot products to show that if the diagonals of a parallelogram are perpendicular, then the length of the sides are equal.
Do not convert to coordiantes to do this. You should work entirely in terms of vectors, so should not mention coordinate anywhere. (The parallelogram could be in any dimension.) Instead, you should rely on the properties of the dot product, e.g., $\u\cdot\u=\u^2$. Remember that your conclusion should be that the lengths of the two sides are equal, so that is where you should end up, not where you start.
Also, start the new WeBWorK assignment

