I'm sorry that this is such a small question, but I haven't done Calculus in years.
I have to differentiate (t-(t^3)/3) and (t+(t^3)/3).
Thanks.
This depends on if the denominator of the fraction contains the variable with which respect you are differentiating. In your examples this is not the case. Remember the fact that $\displaystyle \frac{d}{dx} (cf[x]) = c*\frac{d}{dx}f[x]$. In both of your examples, pull out the 1/3 in front.