Answer
False
Work Step by Step
Consider the following counter-example:
Let $f(x) = x^{3}$
Given $s = -1$ and $t = 2$
See that $f(s+t) = (-1+2)^{3} = (1)^{3} = 1$ and $f(s) + f(t) = (-1)^{3} + (2)^{3} = -1
+8 = 7$
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