kill(all)$

van=facebook.com/LearningCalculus$

f(t, mu, sigma):=(1/(sigma*sqrt(2*pi))) * exp((-1/2)*( (t-mu) / sigma )^2);

fp(t,mu,sigma):=''(diff(f(t,mu,sigma), t));

solve(fp(t, mu, sigma)=0,t);

f(mu, mu, sigma);