[itex]f(x)=5x^3+6x^2-3x+lnx[/itex] (lnx)`=1/x [itex]f(x)=2x^4+3x^2+ cosx[/itex] (cosx)`=-sinx I know that if I only have x, like 3x, then x disappears (correct me if I'm wrong). So what happens with lnx if x disappears? Same thing with cosx. The lesson is extreme values of functions and i saw critical points mentioned a lot and Fermat's theorem, and Sylvester's, but I was told I should understand derivatives first. I have spent the last day trying to understand these, from manuals and internet and I can't find anything clear and simple enough. I used to study arts and psychology and I somehow ended in economics, Please help!!! and please explain as if you are talking to a 6th grade child so I can understand. Or if you can direct me to something where these things are really, really dumbed down so I can understand it. I'd be very grateful.