killl(all)$

atcm2020kill(all)$

;

van=facebook.com/LearningCalculus$

f(x,y,z,t):= 1/(1+2*x+x*y)+1/(1+2*y+y*z)+1/(1+2*z+z*t)+1/(1+2*t+t*y) = 1+k;

limit(limit(f(x,y,z,t),x,0,plus),y,0,plus);

limit(limit(limit(limit(f(x,y,z,t),x,0,plus),y,0,plus),z,0,plus),t,0,plus);

solve(%,k);