If f(a+b) = f(a) + f(b) + 2ab, what is f?
December 1, 2006
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OK, I think I have a proof of the following:
If , then for some real m.
Proof:
Setting and , we have ;
in other words .
Next, set and , which yields .
Since , we have , or
.
Let (the left side)
Then we see that and is an odd function, i.e. .
Now,
but also
Setting equal the right sides of the above two equations and simplifying,
we get
,
which is the definition of linear function with . (y-intercept = 0)
Categories: math