A compound tube is made by shrinking a thin steel tube on a thin brass tube. As and Ab are the sectional areas of the steel and brass tubes, and Es and Eb are the corresponding values of Young's modulus. Show that for any tensile load, extension of the compound tube is equal to that of a single tube of same length and total cross-sectional area, but having Young's modulus of (EsAs + EbAb)/(As + Ab).