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mathematica newbie trouble

• To: mathgroup at smc.vnet.net
• Subject: [mg99276] mathematica newbie trouble
• From: Guapo <yangshuai at gmail.com>
• Date: Fri, 1 May 2009 05:52:31 -0400 (EDT)

i wrote the following mathematica code:

s := (a + b + c)/2;
k[a_, b_, c_] := Sqrt[s (s - a) (s - b) (s - c)];
k[3, 4, 5]
k[5, 9, 12]

when run it, i can't get the write answer. but i change setDelayed(:=)
to set(=), everything works ok

s = (a + b + c)/2;
k[a_, b_, c_] = Sqrt[s (s - a) (s - b) (s - c)];
k[3, 4, 5]
k[5, 9, 12]

i did a google search for the difference of set and setDelayed,
however, i still can't understand it for the upper described problem,
could anyone explain it? great thanks.

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