text: by means of a truth table. prove that a*b =a + b?   but... it isn't. i assume, from your reference to truth tables, that you're referring to boolean algebra, so you're asking me to show that a and b = a xor b. it isn't. in fact , a * b implies ¬a+b, and vice versa.  edit: um... dualspace, what about a=5/4 and b=5? or a=2 and b=2? or in general: ab=a+b (b-1)a=b a=b/(b-1) although, as mentioned above, that's not what he's talking about.