cos \alpha >0
\\ cos \alpha = \frac{1}{4}
" alt="sin^{2} \alpha + cos ^{2} \alpha =1 \\ (\frac{\sqrt{15}}{4}) ^{2} +cos ^{2} \alpha =1 \\ \frac{15}{16} + cos^{2} \alpha =1 \\ cos^{2} \alpha = \frac{1}{16} \\ cos \alpha =- \frac{1}{4} и cos \alpha = \frac{1}{4}
\\ \alpha =(0; \frac{ \pi }{2} ) ---> cos \alpha >0
\\ cos \alpha = \frac{1}{4}
" align="absmiddle" class="latex-formula">