## Algebra and Trigonometry 10th Edition

Let $u=cos^{-1}\sqrt 5$. Then: $cos~u=\sqrt 5$ The domain of $y=cos^{-1}x$ is $-1\leq x\leq1$. But, $\sqrt 5\gt1$. So, there is no $u$ such that $cos~u=\sqrt 5$