The Law of Cosines is a beastly formula, which, yesterday, for the first time in five years, I didn't ask my students to memorize.
I gave them my reasoning: basically, that ten years down the road, ten months, maybe ten days, they'd forget this formula. It's inevitable. I'd rather them pour their guts into creatively operating the formula than memorizing it, since, in the Google era, that's an appropriation of resources I could no longer defend.
At the end of my monologue, I wrote the formula on the board and started passing out tests. One student in the front held up her hand, smiling, the Law of Cosines written brazenly across it.