[tex]\frac{2n+1}{6}[/tex]
Use l'hopital's rule twice,
https://www.google.co.uk/url?sa=t&rct=j ... v5YkwxWyVwor alternatively
Taylor expand all the functions and keep track of terms [tex]1[/tex], [tex]x[/tex], [tex]x^2[/tex], and ignore higher order terms.
Hope this helped,
R. Baber.