Assisting with differentiation and just recalled that I forgot the power of (-1) in the rth derivative of (1-x)^n, r < n in a super valuable question in my combinatorics exam, ffs. It's essentially immaterial to the proof (orthogonality of binomial coefficients) since you only care about the derivative when x = 1 anyway, and the point is that then the whole product = 0, and a factor of +/- 1 doesn't affect that, but it's another dumbass mistake that'll cost some marks in a long list of many. It makes me seriously doubt my exam performance. And the batch I've got coming up are much much harder.