12. a) Show how the thick cylindrical shell can reduce to the case of a thin cylindrical shell with a uniform mass density. b) Show how the thick cylindrical shell can reduce to the case of a solid cylinder with a uniform mass density. c) Show how the thick spherical shell can reduce to the case of a thin spherical shell with a uniform mass density. d) Show how the thick spherical shell can reduce to the case of a solid sphere with uniform mass density. Do not use L’Hospital’s rule.