To simplify the expression \( (x^3)(x^5) \), you can use the property of exponents that states when you multiply two powers with the same base, you add the exponents:
\[ x^a \cdot x^b = x^{a+b} \]
In this case, \( a = 3 \) and \( b = 5 \):
\[ (x^3)(x^5) = x^{3+5} = x^8 \]
Thus, the simplified expression is:
\[ x^8 \]