![\arcsin\left(\cos\frac{7\pi}{8}\right)=\frac{\pi}{2}-\arccos\left(\cos\frac{7\pi}{8}\right)=\frac{\pi}{2}-\frac{7\pi}{8}=-\frac{3\pi}{8}](https://tex.z-dn.net/?f=%5Carcsin%5Cleft%28%5Ccos%5Cfrac%7B7%5Cpi%7D%7B8%7D%5Cright%29%3D%5Cfrac%7B%5Cpi%7D%7B2%7D-%5Carccos%5Cleft%28%5Ccos%5Cfrac%7B7%5Cpi%7D%7B8%7D%5Cright%29%3D%5Cfrac%7B%5Cpi%7D%7B2%7D-%5Cfrac%7B7%5Cpi%7D%7B8%7D%3D-%5Cfrac%7B3%5Cpi%7D%7B8%7D)
В процессе решения я использовал тождество
![\arcsin x+\arccos x=\frac{\pi}{2}, ](https://tex.z-dn.net/?f=%5Carcsin+x%2B%5Carccos+x%3D%5Cfrac%7B%5Cpi%7D%7B2%7D%2C%0A%0A)
а также тождество
![\arccos(\cos a)=a,](https://tex.z-dn.net/?f=%5Carccos%28%5Ccos+a%29%3Da%2C)
которое справедливо, если
![a\in [0;\pi]](https://tex.z-dn.net/?f=a%5Cin+%5B0%3B%5Cpi%5D)
<span>>25 и b> 4 , то ab > 25*4 => ab>100</span>
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(0.06-1.86)+1.05=-1.8+1.05=-0.75