0\; ,\; \; ctga>0\\\\cosa=-\sqrt{1-sin^2a}=-\sqrt{1-0,64}=-0,6\\\\1+tg^2a=\frac{1}{cos^2a}\; \; \to \; \; tg^2a=\frac{1}{cos^2a}-1=\frac{1}{0,36}-1=\frac{0,64}{0,36}\; ,\\\\tga=+\frac{0,8}{0,6}=+\frac{4}{3}\\\\ctga=\frac{1}{tga}=+\frac{3}{4}\\\\2)\; \; cos(a+\beta )-cos(a-\beta )=-2\, sin\frac{a+\beta +a-\beta }{2}\cdot sin\frac{a+\beta -a+\beta }{2}=\\\\=-2\, sina\cdot sin\beta" alt="1)\; \; sina=-0,8\\\\\frac{3\pi }{2} 0\; ,\; \; ctga>0\\\\cosa=-\sqrt{1-sin^2a}=-\sqrt{1-0,64}=-0,6\\\\1+tg^2a=\frac{1}{cos^2a}\; \; \to \; \; tg^2a=\frac{1}{cos^2a}-1=\frac{1}{0,36}-1=\frac{0,64}{0,36}\; ,\\\\tga=+\frac{0,8}{0,6}=+\frac{4}{3}\\\\ctga=\frac{1}{tga}=+\frac{3}{4}\\\\2)\; \; cos(a+\beta )-cos(a-\beta )=-2\, sin\frac{a+\beta +a-\beta }{2}\cdot sin\frac{a+\beta -a+\beta }{2}=\\\\=-2\, sina\cdot sin\beta" align="absmiddle" class="latex-formula">
