0,\; \\t.k.cos20>0,tg20>0\\\\cos(-1)>0,t.k.\; (-1)\; radian\in 4\; chetverti\\sin(-2)<0,\; (-2)\; radiana\in 3\; chetverti\\\\cos(-1)sin(-2)<0\\\\3)\; Formyla:\; 1+tg^2a=\frac{1}{cos^2a}\; \to \; tg^2a=1-\frac{1}{cos^2a}" alt="2)\\ cos700tg380=cos(720-20)tg(360+20)=cos20tg20>0,\; \\t.k.cos20>0,tg20>0\\\\cos(-1)>0,t.k.\; (-1)\; radian\in 4\; chetverti\\sin(-2)<0,\; (-2)\; radiana\in 3\; chetverti\\\\cos(-1)sin(-2)<0\\\\3)\; Formyla:\; 1+tg^2a=\frac{1}{cos^2a}\; \to \; tg^2a=1-\frac{1}{cos^2a}" align="absmiddle" class="latex-formula">
0\; \to \; tga=\frac{1}{\sqrt5}" alt="cos^2a=(\frac{2}{\sqrt5})^2=\frac{4}{5}\; \to \; tg^2a=1-\frac{4}{5}=\frac{1}{5}\\\\a\in (0,\frac{\pi}{2})\to tga>0\; \to \; tga=\frac{1}{\sqrt5}" align="absmiddle" class="latex-formula">