Answer
$$\sec^{-1}12 -\sec^{-1}4 .$$
Work Step by Step
Since $(\sec^{-1}x)'=\frac{1}{x\sqrt{1-x^2}}dx $, we have
$$\int_4^{12} \frac{1}{x\sqrt{1-x^2}}dx =\sec^{-1}x|_4^{12}
=\sec^{-1}12 -\sec^{-1}4 .$$
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