已知函数\(f(x)\)是定义域为\(R\)的偶函数，\(∀x∈R\)，都有\(f(x+2)=f(-x)\)，当\(0 < x\leqslant 1\)时，\(f(x)= \begin{cases} 3-\log _{2}x,0 < x < \dfrac {1}{2} \\ \sqrt {1-x}, \dfrac {1}{2}\leqslant x\leqslant 1\end{cases}\)，则\(f(- \dfrac {9}{4})+f(11) =\)______．