I am having a conflict with two different ways of finding a derivative. Here is the function: y=10*sinpi(.01x-2.00t) Yes, that pi is after sin, but not in the paranthesis. This is how the prof gave it to us. This may be my problem, how I am treating the pi. I figure it was factored out of the parenthesis. So, to find the partial derivative WRT t by hand I do this: y=10*sin(.01pi*x - 2.00pi*t) I multiplied the pi into the () dy/dt = -2.00pi*10*cos(.01pi*x - 2.00pi*t) used the chain rule dy/dt = -20pi*cos(.01pi*x - 2.00pi*t) final result That is my result. I check this in Matlab by entering the following: >> syms x t >> diff(10*sin(pi*.01*x-pi*2*t),t) ans = -20*cos(1/100*pi*x-2*pi*t)*pi So with that I am happy. Now the tricky question. If I enter this same thing to my TI-89, I get: -62.8319*cos(2pi*t - .031416*x) Now...it just hit me that you can transpose the items inside the paranthesis of cosine, and it is the same result. Ok, duh. I don't want to delete everything I just typed. My next question... Am I treating the pi correctly to begin with? Is it correct to multiply it into the () like that? If not, what should I do with it? Is there an easier way? Thanks.