84-1 Differential Equations of Equilibrium in Polar Coordinates Dealing with elasticity problems, what form of coordinate system we choose, which can t affect on describing problem essence, but relate to the level of difficulty on solving problem directly. If coordinate is suitable, it can simplify the problem considerably o For example, for circular wedged and sector and so on, solved by using polar coordinates are more convenient than using rectangular coordinates Considering an differential field PacB in the plate 77 §4-1 Differential Equations of Equilibrium in Polar Coordinates Dealing with elasticity problems, what form of coordinate system we choose，which can’t affect on describing problem essence, but relate to the level of difficulty on solving problem directly。If coordinate is suitable，it can simplify the problem considerably。For example, for circular、wedged and sector and so on，solved by using polar coordinates are more convenient than using rectangular coordinates. Considering an differential field PACB in the plate