Chapter 7.9, Problem 13P

Give algebraic proofs of (9.3). Hint: Write ${\displaystyle {\int }_{-t}^l=}{\displaystyle {\int }_{-l}^0+}{\displaystyle {\int }_0^l,}$ make the change of variable $x=-t$ in ${\displaystyle {\int }_{-t}^0,}$ and use the definition of even or odd function.