Minor correction for 21-260-A recitation 8/27

This morning, on problem 19, I broke up the derivative of t^r into two cases (r = 0 and r ≠ 0). I later figured out that this is actually unnecessary, since we were assuming that t > 0 in that problem and the others in that section of the homework. r*t^(r-1) = 0 when r = 0, and this is the correct derivative of a constant. (There would be a problem here if t were allowed to be 0, since we'd be raising 0 to a negative power, which is not allowed.)

So, you don't have to treat the case where r = 0 and r = 1 separately in that problem, which should make it a bit less tedious.

Also note that the equation in problem #1 was actually linear (I mistakenly listed it as nonlinear at the beginning of recitation). A linear differential equation may have terms that are not linear in t, as long as all terms are linear in y and its derivatives.