kill(all)$

van=facebook.com/LearningCalculus$

f(x):=(tan(x)-sin(x))$
g(x):=x^3$
h(x):=''(f(x)/g(x));

H(x):=''(diff(f(x),x,3))/''(diff(g(x),x,3));

limit(H(x),x,0);

limit(h(x),x,0);