Orthogonal polynomials, Legendre
For
L_1(x) we have the general expression
\begin{equation*}
L_1(x) = a+bx,
\end{equation*}
and using the orthogonality relation
\begin{equation*}
\int_{-1}^1L_0(x)L_1(x)dx=0,
\end{equation*}
we obtain
a=0 and with the condition
L_1(1)=1 , we obtain
b=1 , yielding
\begin{equation*}
L_1(x) = x.
\end{equation*}