(1 ) Divide (g) the path into a number of intervals of△r1,△r2,△r3…,△r…, the element work(元功) △ Wi done by e,over△ ri is given by(3-2): △=F2A=F2C0s日2△i i i i i i W = F r = F cos r   (1) Divide（分割） the path into a number of intervals of △r1, △r2, △ r3…, △ri, …, the element work（元功） △Wi done by over △ri is given by (3-2): Fi  i r   Fi   i