Concept:
Use substitution symmetry.
Step 1: Write \( f(1-x) \).
\[
f(1-x) = \frac{3^{1-x}}{3^{1-x} + \sqrt{3}}
\]
\[
= \frac{3 \cdot 3^{-x}}{3 \cdot 3^{-x} + \sqrt{3}}
\]
Step 2: Simplify expression.
Let \( t = 3^x \), then:
\[
f(x) = \frac{t}{t+\sqrt{3}}, \quad f(1-x)=\frac{3/t}{3/t + \sqrt{3}}
\]
\[
= \frac{3}{3 + \sqrt{3}t}
\]
Step 3: Add.
\[
f(x) + f(1-x) = 1
\]