There is a turing machine M which changes the tapecontent from a^i#a^j to a^(i+j).
construct a turing machine N which changes the tapecontent from a^i#a^j to a^ij by calling the machine M.
forexample. machine m changes aa#aaa to aaaaa. machineN changes aa#aaa to aaaaaa. machine N should call machine M for theaddition.
(informal description of the turing machine willsuffice).