Answer
$$\sin^{-1}\frac{2}{3}-\sin^{-1}\frac{1}{3}.$$
Work Step by Step
We have
$$\int_{1/3}^{2/3} \frac{1}{\sqrt{1-x^2}}dx =\sin^{-1}x|_{1/3}^{2/3}
= \sin^{-1}\frac{2}{3}-\sin^{-1}\frac{1}{3}.$$
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