This video has me brain farting at 4:30 Putting (v1i + v2i = v1f + v2f) in a red box. It should say the equation in the title. I did not use that red box anywhere in the derivation, so all the math throughout the video is good, and so is the final equation. Sorry for the stray equation, but I had a lot of v1 and v2(s) running through my head.