1)t - угол второй четверти, значит Cost < 0 .
В задании опечатка: синус во второй четверти положительный.
![Sint=\frac{4}{5}\\\\Cost=-\sqrt{1-Sin^{2}t}=-\sqrt{1-(\frac{4}{5})^{2}}=-\sqrt{1-\frac{16}{25}}=-\sqrt{\frac{9}{25}}=-\frac{3}{5}](https://tex.z-dn.net/?f=Sint%3D%5Cfrac%7B4%7D%7B5%7D%5C%5C%5C%5CCost%3D-%5Csqrt%7B1-Sin%5E%7B2%7Dt%7D%3D-%5Csqrt%7B1-%28%5Cfrac%7B4%7D%7B5%7D%29%5E%7B2%7D%7D%3D-%5Csqrt%7B1-%5Cfrac%7B16%7D%7B25%7D%7D%3D-%5Csqrt%7B%5Cfrac%7B9%7D%7B25%7D%7D%3D-%5Cfrac%7B3%7D%7B5%7D)
![2)Sinx\leq -\frac{\sqrt{2}}{2}\\\\-\pi-arcSin(-\frac{\sqrt{2}}{2})+2\pi n \leq x \leq arcSin(-\frac{\sqrt{2}}{2})+2\pi n,n\in Z\\\\-\pi+\frac{\pi }{4}+2\pi n \leq x \leq -\frac{\pi }{4}+2\pi n,n\in Z\\\\-\frac{3\pi }{4}+2\pi n \leq x \leq -\frac{\pi }{4}+2\pi n,n\in Z \\\\Otvet:\boxed{x\in[-\frac{3\pi }{4}+2\pi n;-\frac{\pi }{4}+2\pi n],n\in Z}](https://tex.z-dn.net/?f=2%29Sinx%5Cleq%20-%5Cfrac%7B%5Csqrt%7B2%7D%7D%7B2%7D%5C%5C%5C%5C-%5Cpi-arcSin%28-%5Cfrac%7B%5Csqrt%7B2%7D%7D%7B2%7D%29%2B2%5Cpi%20n%20%5Cleq%20x%20%5Cleq%20arcSin%28-%5Cfrac%7B%5Csqrt%7B2%7D%7D%7B2%7D%29%2B2%5Cpi%20n%2Cn%5Cin%20Z%5C%5C%5C%5C-%5Cpi%2B%5Cfrac%7B%5Cpi%20%7D%7B4%7D%2B2%5Cpi%20n%20%5Cleq%20x%20%5Cleq%20-%5Cfrac%7B%5Cpi%20%7D%7B4%7D%2B2%5Cpi%20n%2Cn%5Cin%20Z%5C%5C%5C%5C-%5Cfrac%7B3%5Cpi%20%7D%7B4%7D%2B2%5Cpi%20n%20%5Cleq%20x%20%5Cleq%20-%5Cfrac%7B%5Cpi%20%7D%7B4%7D%2B2%5Cpi%20n%2Cn%5Cin%20Z%20%5C%5C%5C%5COtvet%3A%5Cboxed%7Bx%5Cin%5B-%5Cfrac%7B3%5Cpi%20%7D%7B4%7D%2B2%5Cpi%20n%3B-%5Cfrac%7B%5Cpi%20%7D%7B4%7D%2B2%5Cpi%20n%5D%2Cn%5Cin%20Z%7D)
4^x берем как t и дальше секрет
sina=1-cos^2a=1-49/16=-33/16 sinaна промежутке (п 2п) отриц. sina=33/16
= ( 2а - 2с ) - ( ас - с^2 ) = 2( а - с ) - с( а - с ) = ( а - с )•( 2 - с )