Show that\int\limits_{\begin{smallmatrix}   0\\\end{smallmatrix}}^ {a}{{}}f(x)g(x)dx=2\int\limits_{\begin{smallmatrix}   0\\\end{smallmatrix}}^ {a}{{}}f(x)g(x) are defined as f(x)=f(x-a) and g(x)+g(a-x)=4