### Standard Deviations and… the Law of Cosines?

I'm taking a graduate intro statistics class this semester. I'm not familiar with everything we're doing since I haven't really studied undergraduate statistics. A little over 15 years ago I took a really elementary social science stats class. It didn't cover all that much, and it was a long time ago.

The other day in class we were calcalating… something. I'm not sure exactly what it was. Something standard-deviation-y, but I'm not entirely sure of anything more than that. But the formula looked really familiar.

This is the law of cosines, right? Where the standard deviations correspond to the lengths of the sides of the triangle and the "correlation" (whatever that is -- I'm not entirely sure, it just pops up in the software as a side-effect of calculating other things) plays the role of the cosine? Yes?

The other day in class we were calcalating… something. I'm not sure exactly what it was. Something standard-deviation-y, but I'm not entirely sure of anything more than that. But the formula looked really familiar.

This is the law of cosines, right? Where the standard deviations correspond to the lengths of the sides of the triangle and the "correlation" (whatever that is -- I'm not entirely sure, it just pops up in the software as a side-effect of calculating other things) plays the role of the cosine? Yes?