11. The curve $x^{2}+4(y-a)^{2}=4$ intersects the curve $x^{2}=4 y$. The range of values for $a$ is .。



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11. $\left[-1, \frac{5}{4}\right)$

Let $x=2 \cos \theta, y=a+\sin \theta$, from $x^{2}=411$, we get $a=\cos ^{2} \theta-\sin \theta=$ $-\left(\sin \theta+\frac{1}{2}\right)^{2}+\frac{5}{4} \in\left[-1, \frac{5}{4}\right]$.