D-11. Series of the form m 3(1) = Z :11" \"=0 occur frequently in physical problems. To nd a closed expmssion for Six}, we, start with I = i I" 1 x ":0 Notice now that S{x) can be expressed as d " ,. \"3 ,. x a I; x _ E m: and sham that 3(x) : If\" - x)