Algebra explored what the side lengths are for a rectangle with area x² – y² today. The paper folding required to get there was a lot of work and I am not sure if many of them totally understood what was going on.
Either way it is good for them to see a visual representation of where the heck these things come from.
Geometry practiced Law of Sines and Cosines, which we spent yesterday deriving. I talked to them quite a bit about how parallel these are to calculus in that they are using geometry to set up the problem then algebra to solve for either the side or angle. To me, that seems a lot like how calculus is; use an integral or something to set up them problem but after that it is all algebra to reach a solution.