I tried to solve this exercises, proposed in 'A Mathematical Introduction to logic' by Enderton §2.7 exercises 1.
Assume that $L_0$ and $L_1$ are languages with the same parameters except that $L_0$ has an $n$-place function symbol $f$ not in $L_1$ and $L_1$ has an $(n+1)$-place predicate symbol $P$ not in $L_0$. Show that for any $L_0$-theory $T$ there is a faithful interpretation of $T$ into some $L_1$-theory.
But I can't solve this problem. Thanks to for any help.
Edited : I add a definition of faithful theory.
Let $T_0$ and $T_1$ are theories (in a possible different language) and $\pi$ is interpretation of $T_0$ into $T_1$ then $\pi$ is faithful iff $\pi$ satisfies $$\sigma\in T_0 \iff \sigma^\pi \in T_1$$ where $\sigma^\pi$ is formula interpreted by $\pi$.