kill(all)$

van=facebook.com/LearningCalculus$

a(x):=(x + x^(2/3))^(1/2);

b(x):=log((1-x^2)/(1+x^2));

n(x):=''(a(x)*b(x));

d(x):=x^(7/3);

f(x):=''(fullratsimp(n(x)/d(x)));

limit(f(x),x,0,plus);