Re: Help with Identities

*To*: mathgroup at smc.vnet.net*Subject*: [mg65592] Re: Help with Identities*From*: "david_tang" <david3687 at gmail.com>*Date*: Mon, 10 Apr 2006 02:31:12 -0400 (EDT)*References*: <e1ahdv$2j1$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

dear sir the reason maybe Mathematica does not know the x and y greater than 0 or not. so you must assuming the x and y greater than 0; and more over, the log[] doesn't smarter than man. so you must tell the Mathematica to do it using the function FullSimplify or Simplify; and then, FullSimplify[Log[x/y] == Log[x] - Log[y], x > 0 && y > 0] NOTE is two == and return true; but if you use === ans it will return false, because the header is different i think; But if you use this Assuming[x > 0 && y > 0, Log[x/y] === FullSimplify[Log[x] - Log[y]]] telling the Mathematica the x and y greater than 0 first, you can get the result true both use == and === i hope this will hope you. you also can test Sin[2 x] == 2 Sin[x] Cos[x]; without Simplify[] you will get nothing.