Greetings- out of college for 50 yrs and studying H.M. Schey's book. Cannot understand his derivation of the z component of the curl of a vector function F for a part of a sector of a circle in a plane parallel to xy axis. Cylindrical components. Let me describe equation as three parts for ease in reading. Eq. is a(b-c). a= minus delta r/(r (delta r delta theta)) b= F sub r of [r, (theta plus (delta theta/2 ),z](delta r) c= same as b and change plus to minus. Author takes the limit of the entire equation as delta r and delta theta approach zero, and has an answer of minus (1/r)times the partial of F subr with respect to theta. I understand everything except how to take that limit. Can some one help? The example is on page 83 of Schey. Thanks, Jim.