20. A ladder is propped against a wall making an angle O with the floor as shown in Fig. 5. The wall is frictionless but the coefficients of static friction and of kinetic friction between the floor and the ladder are /, and /, respectively. What is the expression for the smallest angle O if the ladder is not to slip? smooth rough, with coefficients (us, /k) Fig. 5 A tan-1 x 2 tan" C tan -1 D tan" tan-I